Field guide t o halobiid and monotid p l e c p 8 s

of the Alsskan Tr iass ic

Among the rczrine inve&ebrate negePossils i n the T r i ~ s s i c rocks of Ahska, halo%lid and monotid pelecypds a re the mst useful stratipaphic indices. By a large nargin they are the ziost widely d b t r i b u t e d of Triassic negafossils In Aiaska, and t h e y s r e r epesen ted by species v i t h f a i r l y r e s t r i c t ed s t r a t i g r ~ p h l c ranges. LmonLtes i n mst czses provide mr.e ref ined s t ra t igraphic ege ~ssigruaents, 'but Kith a f e w notz31e exce?tions a m o n i t e ftiunas a re scarce i n the Alaskan Triessic , and where present they are generally accompanied by halobi ld or ~ o n o t i d pelecypds.

I n addition t o t h e i r velce i n subdividing Yddd3le and Up?er Tr iass ic s t r a t a , the halobiid genera Dzonella ~ n b Halobia, an6 the monotid genus ~ o n o t i s , pro- vide e rea* mans of r e c o p i z i n g beds of Pr iass ic age. T.ese genera are r e - stricted t o t h e Trlzssic , end reasonably well-preserved examples are no t eas i ly confused with other r e m t e l y related f o s s i l pelecy-gods of older o r younger system.

Tbe i c t en t of t h i s f i e l d mide i s t o characterize the genera Daonella, Ralobia, and Monotls, t o i l l u s t r ake represe&ative species from t he Tr iass ic of - ,. . Aleska, and, i n s o f a r as possible, t o provide information on t h e i r - s t r a t i ~ a ~ h i c d is t r ibut ion . A t present, howsver, the l a s e r two of these objectives can be o n l y partly f u l f i l l e d . In s, rimber of casqs the specimens avaibble for ills- t r a t i o n a r e inadequate, t h e s t rat igraphic ranges of the various species are un- cer ta in t o varying degrees, and the t a o n o r q of these pelecypods at t h e spec i f ic l eve l i s largely unsettled. Concerning this l a s t paint, t he specff ic nonencla- t u re Kithin t h e important g e n s Ralobia i s especially confused. Well over 100 specif ic nanes have been proposed for Ealobias from different parts of t h e world, but mny of these nanes are poorly established, and a la rge proportion of them wlU. upon -her s t u e probably prove t o be synonomous. A s a consequence, the speci f ic names used hereic are subJect t o d r a s t l c change.

Tnus, although the present understanding of halobiids and mnot ids malres them u s e m i n a general my, zuch rez i ins t o be learned abourt t h e i r taxonomy, strat igrayhic dis tr ibut ion, and evolution. HopeFully, t h i s prelwnary f i e l d guide will not only be of some inmediate value but w i l l I.ncite f i e l d p r t i e s t o co l lec t s t r a t i p a p h l c a l l y loctited smples which can be used i n preparing a more Eef i n i t ive and better i l l u s t r ~ t ed trestment of these pelecypoas for eventual p~blictxt ion,

Table -1 shows, ~,.',th perhaps not l iberal enough sprinkling of question narks, the lmom stratigrap3ic ranges of the various f o m illustrated.

Tne cheracteristic noryhologic f e ~ t u r e s of Daonella, Halobia, and Monotis Eye shown iilzgrmatically on p l ~ t e 1. Eech of tine figures on t h i s plate repre- ser-ts a ri@t valve, end the ~ p n l f c a t i o n of conventional descriptive terminal- o i ; ~ t o these s h e l l s is indiczted on f i g m e 1 as follows:

H = height of shel l

L = lergkb of shell

d = Corss l r8rgf.n or hjllge l i n e

a = anter ior m g l n

abditioL to these t e r n , t h e i n i t l ~ l pa r t of t he she l l a t the point of origin of the rac5ial ribs i s the "mko" o r "beak, " and . inequi la t era1 shells ~ 5 t h the beek located an te r io r t o the mid-len&h of the shell are t e m e d "oblique." "&rsl' m e &eveloped on e i ther side of the beak where the dorsal part of the shell i6 differentiated from the r e s t of the shell s ~ ~ f e c e . Depecding on their cocvexity, shells range f r o m " f la t" t o "inflated." Z e f i and right valves of "ec_uivalved" s h e l l s are elike i n t h e i r convexity, s t rdc ture , and omanentation. Concentrically arranged folds represeriting growth stages are termed "rugael1 (sing. "xuga").

. The size of f'ull-grown individuals i s a potentially stable character of - each species, but generally no indepencient Zeans i s at hand f o r d i s c r i d n a t i n g =ture shells from &nature ones or for reco@;nizing environmentally dwarfea sbells . Saell size is thus based on negative empirical evidence and must be used with caut ion i n making iden t i f i ca t iong , As halobiid and monotid pelecypods hsve s imi la r s i z e ranges, the species illustrated herein ' are characterized by maximum shell height as follows: less than 25 mi, "snsll"; 25-50 m, "nedlum"; and more than 50 mm, "large." All illustrations on plates 2-6 are natural size.

Locel i ty nuhe r s r e f e r t o USGS 2b?esozoic locality registers e i t h e r in Menlo Park, for cumbers prefixed by the letter "M", or in Washingkon, D. C.

Pnotogrepby of specimens from t h e US@ c o l l e c t i o n s is by KenJi Sakamoto.

Shell flet, equivzlved, nearly eq -dh te r a l t o oblique. Dorsal z r g i n elon- e t e and streight. Radial r ibs stra;ght, r a r e l y wzvy. Conceatric ~ o w t h l l n e s or mgae weakly developed, nost c~nsgicuous near beak. Ribbing extends to dor- s d margin; ears not present. See pl. 1, f i g . 1.

Shell longer than high, slightly oblique. Ribs on cent ra l prt of she l l r ek t ive ly wise snd flat t opgzd, 3-4 t h i e s wicith of i n t e r s p c e s , pzdually sub- aviding i n two ' s and three1 s.

Age: Ladlnian.

Deonella cf . - I). s-ibquz&&ta Yabe and Snimizu

71. 2, f i g s . 7-10

Shell evident ly nearly as high ss long, nearly equi la te ra l . Ribs narrow, not mch wLder than i l i te rspces , =re o r less even i n strength ~5th gradual 2- or 3 -fold s ~ b b i v i s ion.

- Age: fadinian. ---. - - -- - - Remarks : Tne avsi lable specimens a r e inadequately preserved t o accurately

c h m c t e r i z e t h i s species, but it is clearly d l f f e r e ~ t from D. frami with which - - it OCCUTS. ,.. -

Genus FfiOBIA Bronn

Shape l i k e Dzonella; sbeU I l a t , equivalved, more' or s t r a i&t , elongste dorszl -gin. I n some species r a d i a l &cross s h e l l SL-face t o the vent ra l a r g i n ; i n others a dj

l e s s oblique, with r i b s run s t r a igh t . s t inc t i n i t i a l growth

s t ~ j i e i s delimited by unifo,m, an ter ior ly concave curvature of the ribs which ends with abrupt recuruetuz-e st a ruga near the d o . This i n i t i a l growth stege i s followed i n some species by one or more successive concentric g-owbh s t sges wlth curved ribs, giving ribbing E regulerly wzvy aspect. Concentric growth striae o r r u e e developed t o varying degrees. Shel l surface smoth near anter ior Corsal m r g i n forming a t r ianguls r a t e r i o r ear, sometimes elevated above adjacent ribbed prt of ,shell or separated from it by a groove. Ribbing fades towards posterior dorse.3. =gin Tonning a porlycdefined poster ior ear. See pl. 1, fig. 2.

2:. 2, f igs. U-15

,%ell zejium Size, longer than high, s l igh t ly oblique. Ribs strai&t or sently cmte?, across z s s t of s h e l l height. On central part of shell second and thir& orcier r i b s are ' I r r e g d ~ r l y ~ o u ~ e d together i n t o se-reral broad p r h a r y ri3s S e ~ z t e b by p r o d s e ~ t i n - l e r s ~ c e s .

Age: k t e Karnian.

~ e s r i r s : Smith (1927) propsed the rime R. brooksi for f o m like thts fk.03 the IJrangeU E 5 t s . with r e l ~ t i v e l y sbwp c=ature of the ribs near t h e ven- t& =gin, but t h e p o p r i e t y of this diskinction has not been' ciemonstrated.

71. 2, figs. 16-20

S m U o r mdium s:ze, longer than high, s l i g h t l y oblique. Fine r i b s of mze or less e q ~ l s t rength d5stribzted evenly across cen t ra l part of she l l . Dis t inc t i n i t l a l gro;r',h stage of curved ribbing.

Age and r e z r k s : Rep1te6l.y widespread i n rocks of l a t e Karnian ege, but

€3 ~ J J Y of t'se references t o E. "si;?er;52" i n the l i tex&xre probably r e f e r t o the earw ~ o k % h stages of o%i& species t h a t have d i s t inc t ive ornamentation when fu l ly grokn. it seems likely, however, that smll forms with a r e l a t i v e l y broad initial zone of curved r ibbing end with ribs-of equal strength across mst of shell s u f a c e my be e d i s t i n c t species likk H. superba which is perhaps ances- t r a l t o sone sinilar bk nore cozp1exI.y o m e n t e d species l lke H. cordillerana. Tne s t ra t igraphic r e l a t i o n t o sbi lar species I s unknown f o r the-southeastern Alaska S ~ ~ C ~ E S ~ S i l l u s t r a t e d here; i n the IJrangell M t s . s h e l l s of t h i s kind occur in the loKer psrt of the Ch i t i s tme Limestone. Ralobia " z i t t e l i Llndstrom" of Tozer (1961) a y correspond to t'nis species.

Fielobiz o r ~ t issins. Smith

Pl. 3, f i g s . 1-3

Shell xzefiium t o large s i ze longer than high. Strong, flat-topped prFmary r i b s bear 2, or sonet&es 3 or 1, much wezber secondary ribs. Ribbing fades in strength and becomes I r regular ly ~ a v y on posterior half of she l l . Umbonal zone sharply s e t off by abrupt r e c~ rvs tu r e of ribs.

Age: U t e Karnien; s t r e t i ~ a p l i c r e l a t ion t o other species present ly u;ijcnown.

~exrar'ks: Ealobie gigzntea S d t h anC H. " z i t t e l i Lindstrom" of FZttl(lg07) end I403 S ~ S O V ~ C S ~ are pssibly conspe?dic.

Belo'oie cordillerena Sni th

?lo 3, f i g s . 4-10

She l l rjedium t o large size, longer than high. Ribs neer beak rarrow, closely spced , end of more or less even s-lren@h or with tendency toward a l t e r - cat ion of ~r- with wezker seconhry i n t e r s p c e s . Distinct umbonal zone of c m e d ribbing, followe8 on anteeriol- prt of s h e l l by severa1, less d i s t i n c t con- cent r ic zones defined by changes i n direction of ribbing. Ribbing progressively decreases i n strength and Secozes i r r e m r l y wavy on poster ior two-thlrds of adult shell.

T o Age: & t e s t -mien M. ear ly Norlan ( 7 ).

Remsrks: I n add i t ion t o the l o c a l i t i e s represented by i l l u s t r r t e d s$eci- mens, t h i s species occurs i n the l ~ w e m s t 20 feet of the XcCzrthy ??omtion, just above the occurrence of Rerotoceras, East Fork of McCarthy Creek, WrengeU. It 6 -

Hnl&ie dd l iena Smith

PL. 4, f i g s . 1-4, 5-81

Shell r ed iumto lzrge size, g ~ o p r t i o n s var iable , lengLh e i the r grea ter or l e s s than height. Dorsal z r g i n r e l s t ive ly s h o d . Radlal ribbing straight, In t h e form of f ine , somewhat k.avy stria-lion of uneven strength. Umbilical zone ofs curved r ibbing p o r l y developed or absent. - - -.

Age : Uncertain; possibly early Norian.

Remks: Halobia s e ~ t e n t x i o n a l i s Smith and H. syrumetrice. Smith were origin- a l l y described f r o m t h e sene population sample and are regarced as variants of

Eelobia cf. H . fallex MoJsisovics - Pl. 3, f ig s . ll-14, 157

Shel l small, longer than high. Fine, closely-spacedJ f l a t -topped r i b s curve& an te r io r ly over rcost of shell surface. Closely s p c e d rugae on d o n a 1 region pronounced but unwen I n strength.

Age: Early Norian.

Remrks: I n adaition t o the i l l u s t r a t e d specimens, t h i s species i s known fro= the Pt. Hope s rea of northwestern Alaska iiterbeaakd with ~ o n o t i s spp.

11. 3, ~ l g . ~6?; 21. k , f i g s . 9-io, 15

5 ! ? 2 5 : 3 t o lerge s ize ; e lar ,~&e ier_&ln.k;is~ t o r,early eq: l lz ter&l ir shzpe. Closely-~~&ced, f -topp:i s.: r5igLt rybs h<t> tendency kc\=:& r e g d e r . ' , z se~ ion of S ~ C O E & ~ ~ ir.terspces so t k ~ z eeck ;r-y r i b ten& t o 3 e u 2 seconkry r ibs . Cor?cen'-Lric strlee end r u s e u s u z l l y &istinct.

Age: Tarly L,o m i d 6 l e Fories.

Re-r-rks: S-oecinens desc;-11?& by S s t h (1927) es 5. - ' r ?~ lo r i ce l4o~sisovics snd E. e l s s k ~ n z Sr5th e re >ro\risionzzy incl .~deZ m6e=. tkis LEE, bc% m r e t'n.%L - - one species ~y well be ne:&C ::ere.

L Shel l s *th sixLler s k a - , ~ szd 0 r z 5 e 2 + , bz% i . s t h o , ~ t ~ e l l - ? e f f i ~ . ~ L 2zteric1. o r ~ s z e r i o r ears occm zt ~ b c ~ u t %be szce strztig~$.i.c I_s:rel i n t L e 425s r.i:;e-e t h e y hzve -=?en r ~ x e d ?eor?_e:l.s ir.:?rl~lis K i t t l ; s l d l e r form z y z l so oczxr 25 scl-lhenz Celi?'orr,fe end. e e z t z r n Czegcn and right b e expected in Ales-. m e r e - l z t i o n of l k o z e l h t o Ezlo3ia i s Ycus zcre c o ~ g l e x then z six21e c n ~ n g e nezr t'le begcaning of k t e Triassic t5z.e I r ~ a e=less t o esxed haiobiids.

- S3eI.l s r C L , height enb lengkh &bout e w l , s l i g h t l y oblio_ue. 3ibs s t re ight , very fine, of Even stren&r,, enii r e g d m l y spaced with a density of zbo-at 5 per- -- -

. Co~cenkric rugae w e l l developed.

Age: ezrly t o & d u e Xorian. ,+

Shell Ineqc5valved; l e f t valve nore iL3zted than'righ%. Inequilzteral 22s oblique, ger,erelly ovoid i n shzye ~ 5 t h p s t e r i o r rmrgir, mre brozEly ro-mded tksn ~ r i t e r i o r ~ r E ; , n . Anterior ~ r t of posterior dorszl ~ r g l n developecl in to e gos- t e r i o r e s r ktiich i s =ore or l e ss b i f feren- l i~ ted from t 'ne r e s t of the shel l . Anterior 2 s r e p ~ e s e n t e d by szall spm?-like structure beneath be& of r igh t velve only. 3ibs generz1I.y nerrcV zn6 sharp; different orders cf rlbs cozzonly differ ~ ~ r k e d l y ir, stresgkh end tend t o zl-lerzate i n position. Closely end reguler ly speed concentric s t r i a e co~spicuous i n some species.

I ~ l o ~ o t i s s c ~ t i f o ~ s cf . 14. s. tmicz. Kip r i sova . - -

21. 5 , f lgs . 1-2

- S t o zea l - c s i z e . ~ o s t s r i o r e&r poorly d l f fe ren t iz ted , ckiarzcterizfii o ~ A y by ~,brxtjt c ' r ~ c g e f r o = the csr,ve~Ay romded o - i l t l i ~ e 0;" tke pos-lerior Lo-szl rzrgir! t o t h e s-,r&-i@t k:r_,e lire; r:-~-~ing on s'nell surfece continues ozto e c . 3 9 s mzrov, closely ~ p c e , ; 5% of iSYexent orders ebol;t equal in st resSh.

. Rezrks: Tne E V ~ ~ ~ E I L E , secon-ily f la t tened , P J a s b n 6pecin;ens IncluZed m&er this = m e Ere ir;sufZl~ieri-l fo r zdea_uzte cbrecter izs 'c ion.

Tlefiiu size; height o: z~cst shells 80-9@ of lenGh; l e f t vzlves very Zrcz s l i @ t l y t o mch m1"e in"-lzed t h m r:&t valves. Posterior e a obtusely tm- czteed poszerlorly, generelLy dlfz'erez%i,~ted from poster ior iiorsel m r g i n by o d y sl:$t conczvity Ir, s h e l l cutl ine, c ~ ~ n l y bezrs fine ribs. Ribbing on shell SL-fece dense but ver izb le i n s t r e n ~ h zmng she l l s of e single popul~t ion; e x u t of coxtrest In strength be;-$een diZfcrent orders of ribbing a lso varizble. Con-

1-ge: Eerly Korlzc. - - - l i e ~ r k s : C ~ r z d i a n s~eci~ens I'xcz~ the " H i ~ v a t i t e s zoae" referred t o

M. - " z l z s 1 ~ ~ e S ~ t h " by McLezm and by Tozer probably belong t o M. s . pinensis r s - - pointeeb ol;"c by 1Testemn.n (1562, p. 759). ,,

PI. 5, f ig s . 15-17, IS?-; pl . 6, .figs. 1-4

k r g e s ize; left valve o r ly m6ere t e ly mre inf la ted than r ight valve. Pcs- t e r i o r e z r w e l l d i = ' r ' e r e ~ t i ~ t e d , unrijbec, truncated poster ior ly by pronounced c ~ a - czvi ty i n shell outline. ?adizl ribs sherp en& mrrow; i n t e r s ~ c e s relat2vely wide zn6 f l z t . Dig' , ~ e r e n t order ribs tend t o alterrate r e p l z r l y so tha t a re la- tively x-ezk seconkry r i b f ~ ~ e r v e n e s between each p z i r of p r m ribs, end a s t i l l t;eelier third-order rib I~%ervezes i n t l n n between adjacent first- and secoc2-order r'bs. Concentric g a ~ E f i striae regularly and closely s9ceci, con- sqicuoas on outer shell surf~ce.

Age: hEddle o r l&e ircrisn.

3e-lrks: hall., stror-gly in-?leted l e f t valves l i k e %h&t i l l c s t r ~ t e d by ~ 1 . 5 , ZLg. 18 o z c - a with >'.. sz;';elrz-Lerls i n t h e hTrengell Nt s . ; ;,%ether o r not -. - - they ~ ~ r ~ ~ o l o ~ i c a l l y intercede >:it3 >I. siiocircELaris i s uzlcerteln. -

Monotis s a l i x ~ r l a cf. M. - - 6. salinaria Bronn

[= Monotis el~skana Smith]

PI. 6, figs. 5-8

Kedium size; elongate lengthk-ise; height only about 70$ of lengkh; left n l v e only s l i g h t l y more inf lated then right valve. Posterior ear weU differ- ent iated and smooth; posterior t m c z t i o n angle of ear approaches go0. R 2 d i a l ribs nunierous, narrow, and closely spced; weak secondary riba tend t o inter- vene regular ly between stronger p r h r y r ibs. Fine concentric powth ~;trLae regularly and closely spaced.

Age: k t e Norian.

Monotis sa l inar ie cf . M. s. haueri K i t t l - -

Size and shape of shell and cherecter of poster ior ear l ike t h a t of M. 6 . c f . M. 6 . salimria; ommentation also similar, but closely-spaced radial - rib; vider-anx l e s s nuzerous.

Age: Iate Norian.

Nortot is ochot i c a ( ~ e ~ s e r l i n ~ )

Medium t o l a rge s ize. Left valve strongly inflated, riat valve nearly flat. Fosterior esr well different iated and smooth. Radial ribs coarse and relatively few i n number. Different orders of ribs Wferent ia ted by size, tend t o a l t e r - nate reg.ularly. Concentric s t r i a e poorly developed.

.. .. Age: Middle or l a t e NorLen.

R m k s : In Alaska M. ochotica s.s .- has been. fo&d only i n the Brooks Range, and its s t r a t i p a s i c r e l ~ t i o n t o other species of Nonotis i s not known.

K i t t l , Emst, 1912, Yffiteriales zu e b e n Monopaphie der IlaloSlidae u~rld Monoti&@ der Tries: Res. K i s s . Erforschg. k la tonsees , v. 1, pt. 1, h 2 o n t . Anhang, Y. 2, no. 4.

3503 sisovics, E . von, 1874, her die triadischen Pelecypoden-Gattungen %onelLe ur;d Helobie: Ab'handl, geol. Reichsanst. Wien, v. 7, pt., 2 .

Wth , J. P., 1927, Upper Triassic mrine invertebrate faunas of North America: . U. S. Geol . Sumey Prof. %per 141.

Tozer, E. T., 1961, Triassic stra%igxiphy and fsunas, Queen Elizabeth I s l ~ n d s , * Arctic Archipelago: Canads. Geol. Survey Mem. 316.

hTestemnn, G. E. G., 1962, Succession end variation of Nonotis and t h e associ- eted fama i n the Norian Pine River XSridge section, B r L t i s h Colurrbia (e iass ic , ~elecypoda): Jou r . Paleontology, v. 36, p. 745-792, pls. I.E- 118.

North American

J IIRILS SI

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- .-- ---

C+oris toccras marshi

Si rcn i t c s argollautao

Pinaco ccras mcttcrnichi

C p t o p l c u r i t c s b icr cnrt.l;u~

Cladiscitcs ruber

S a g e n i t ~ s gicbcl i

Diccophjllitc3 patcns -

T r o p i t c3 subbullatus

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Nathors tit es Pro trachyceras ar chelaus

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Protrachycerm r c i t z i .

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P~racoratitcs binodosus '4-4

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Italoritcs

I l h ~ v t l t i t c s

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Xcopop.? noccras haugi .------. * .---- A-

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Pla te I

v Fig. I - Doonello

Fig. 2 - Holobio- .*

Fig. 3 - Monotis

Pla te 2

i7Fgzes 1-6. Ikonella frmi K i t t l . - k

1-2. h c . M1790, si l iceous shele beneath Nigolai Greenstone, Skolai Creek, Wrangell Nts.

3-4. Loc. m395, "lower part oi. ~ h c b l i k ~ m . " (BP -1. CO. ), Ikiapuk Creek, Canning River area, Brooks Rg.

5 . Le% valve copied from Tozer (1961, pl. 29, , fig. 3 ) ; Schei Point Fm., Table Island,Cenadian Arctic.

6 .. . Toptype r igh t valve copied from Tozer (1961, pl. 29, f i g . 1); Blaa Eiountain E'm., lower shale neziber, B k a Mountain, Ellesnere Island, Canaciian Arc t l c , ,

7-10. Daonella cf . D. subqudrata Ycbe and Shimizu. - k

7-9. Loc. Nl790, s i l iceous shale beneath ITifolai Greenstone, Snolai Creek, Wrangell ERs .

10. LOC. 1k036, approximately eq~valent to Ml790. mote frag- llient of D. frami at bottom of figure. - -

11-15. Zalobia austrieca Mojsisovics.

Loc. MI-709, lower part of McC~rthy Fm. (? ), Canyon Creek, ITrengell M t s . An abnormally high ~ l @ t valve; associ- a t ed with Anatropltes .

12-13. Right valve from loc . Ml794 and l e f t valve from loc , KL792; Nizina Lime st one, near Bonenza Peak, Wrangell xts.

14-15. Lef t and right valves; loc . 8153, basa l McCarthy Fm. (? ), Kushiiana area, Vrangell Xts. Type l o c a l i t y of H brooksi Smith. -

16-20. Ealobla cf. R . s u s r b a Moj sisovics. - 16. Loc. ll237, Keku Straight, southeest Aleska. Associated

wlth Pzratropites.

1 . k c . 10197, K e h Straight, southeast Alaska.

18-20. Loc. h240, Keku Straight, southeast Alaska. Associated with Discotropites.

Plate 2

P l s t e 3 - l i g z e s 1-3. Ealobia o r n e t i s s i m Smith.

Loc . 4823, Keku Straight , southeast Alaska.

'. 4-5. Uni"lz3tened irmnsture shells; loc. ML710, 20-30 feet

strat i g a p h i c z l l y ~.bove " D i scoyhyllit es limestone, "

spur between Ration =d '1:~'Iroc Bivers, Upper Yukon.

6-7. Loc . ~ 6 7 6 , West Fork Ivishek ~ i v e r , Phillip Smith Ntn. 1: 250,000 qued., 3rooks Rg. Stratiigrphic position

4 unknown.

4 8-10. Loc. mFfl3, a f e w t e c s of T'eet s t r z t i g a ~ h i c & l l y above " D i sco-ohvllites llziestone, I' Trout Creek, Upper Yukon. Approx-tely s t ra t igre?hiczl ly equivelent t o type l oca l i t y on southwest b n k of Yukon River near Nation.

ll-14. Halobia cf . H . f allax KO J s i sov ics . - 'U U-12, 14. . Loc. ~1714, a few hambed f ee t s t r a t i g reph lce l ly above

t I ~ i s c o ~ h ~ l l i t e k limestone, " Trout Creek, upper Yukon. ---

Associzted wTth Mo~otis scu t i fomis pinensis, a fragment of which appears on t h e l e f t side of f ia . 14.

13, 151. Loc. 9383, soutkwest bznk of Yukon River Gear Nation, Ugper Yukon. A few h u d r e d f e e t stratigaphically above "Discopk~ l l i t e s - limestone" accori ing t o Black- welder (1915 notebook, p. 26-27):

" 16. 7I I~ lob ie cf. E. - d i l i t a t a K i t t l .

h c . 13425,' southwest bank of Yukon River near Kation, Upper Yukon. Severel hundred feet strzt igraphical ly above "Disco$yllit es l inestone " eccor&lng t o Merti'e (1925 notebook, P= 109, = b ) =

Plate 3

h c . 10196, Kelcu SLraigkt, s o ~ t h e a s t Alask~; strat igraphic p s i t i o n unknokn.

Loc . mn8, Michigan Creek, U p p e r Yukon; geogre@ically close to e x p s c r e s hdtk 3'i~notis sp.

9-10, LOC. 9533, Grzvins Islacd, sout'neast Alaska; t y p e locality of E . e7kslknz Sai th . -

15. M c . 32698, &bout 200 feet s t ra t igx@xtce l ly ~ b o v e bese of 14cCarthy Fa. 2nd zbout 50 feet be107 lowest occurrence of l o n o t i s subcirc?;leris, Ees t Fork of McCuthy Creek, Wan g ell s .

12-14. Loc. ~ 1 6 9 7 , 30-100 feet stretigraphicelly above bese of .. McCarthy h., Ezst Fork of McC&hy&eek, Wrangell M t s .

.. P la te 4

~:~x--es 1-2. Elonotis scutiforrcis c f . ?'.,s. - ,nica Kipr isovz . - - \ 1 Left valve, .lot. .El02G, aSout 20 feet s t r a t i g a p h i c a l l y

zbove "Dlsco~hyl l i tes l izestone, " Trout Creek, U p ~ e r Yaon. ~ ~ ~ r o ~ i ~ t x ~ e q u i v a l e ~ t t o loc . Ml713 with Halobia c o r d ~ ~ e r e n ~ .

~iat valve, ~ o c . 8896, so-~t'nwzst b&nk of Y&on River new K ~ t i o n , Upper Yukon. About 3 0 feet strzt igrephically ~ b o v e "Discophyllites lir;estonetl accorang t o Yartin (1914 notebook no. 2, p. 18). Eelobia cordillerane. and T r o a i t e s cf. T. uelleFl included i n collection fromthis

, 3-14. ITonotis scu t i fomis cf. 15.s. - - ptnensis Tr'estermnn'.

--.+ 3-8; b c , ,+Tl5,,, .+ ,few hundred feet s t r ~ t i g r a p h i c a l l y above

"Discoyhyllit es lbes-Lo~ie, " Trout Creek, Upper Yukon. ' 10-30 Test higher thar, loc . a714 Kith Helobia cf. H. - fellax. Figs. 3-4, right v ~ l v e s (note anter ior eer on fig. 3 ) ; 5 , i m t u r e l e f t and r1gn.t valves; 6-8, wt \t{r valves showing variat ion i n convexity.

9. F k t t e n e d "double imge" shokdng ex+esior of ri&t valve ~ n d in te r io r of correspnding l e f t valve. h c . ~1716, apgrox- i m t e l y t h e seme es loc. M1715. Spechens associated with

- -. Himvat it es? sp. - .:lo-14. Loc. ~ 1 3 9 8 , "40 f e e t belokT t o p of 270-foot section of Shublik

Fm." (BP Expl. Co. ), Sdle roch i t Mts., Brooks Rg. Figs. 10- 12, lert valves; 13-lb, right valves showing anter ior eer.

15-17. Monotis subcircular is Gabb.

15. Lef t valve (top) and p a r t i e l right yalve (bottom). Loc. 10690, 280 feet cbove base of McCarthy h., East Fork of McCarthy Creels, Wrengell Mts .

16-17. Right and l e f t valves. Loc. KL701, a f e w hundred fee t a"9ove base of McCarthy Fm., "Monotis" Creek, Nizina River area, kTrangell Mt s .

Strongly inflated smU. l e f t valve from same bed as figs. 16 -17 above.

Plate 5

F w e s 1 - Monotis ~ ~ b c i r c d a r i s Gzbb . Loc. ~ ~ 6 9 0 , 280 f e e t ~ b o v e bese of McCarthy h., East Fork

of McCerthy Creek, I.Trer,geil 1Gs. Figs. 1-3, r ight valves; 4, l e f t valve.

5-8. lb50notis salinarie. cf. M. 6. sz i i~~er i e . k o n n [= M. alaskana Smith]. - - -

Lot. 1~692, 515 feet ~ b o ~ ~ e bese of IIcCarthy Fm., East Fork of McCzrthy Creek, Wrangell ~ t s . ~ i g s . 5-6, left valves; 7-8, right valves.

9-U. I4onotis s a l inz r i a cf . M. s . h~uesi K i t t l . - - Loc. 1Q693, 575 fee t ~ b o v e bese of McCari;hy Fm., East Fork of

McCzrthy Creek, k%engell ivits. Fig. 9, l e f t valve; 10-12, right valves.

12-16. Elonotis ochotica ( ~ e y s e r l i n ~ ) . 12-13. Left end r i g h t s ides of sece specimen; copiedfrom Tozer

(1961, pl. 30, f i g s . b-4b). Eeiberg Fp., EUesrnere Is- Lend, Csnadian Arct.ic .

14. Left valve - copied from Tozer (1961, pl. 30, fig. 8). - .

Beiberg f@. , Cornwall Ielenb, Cenadian Arctic. -

-15-16. Loc. ~1453, "top of Shubl-ik 4%. with t o t a l thickness of 330 feet" (BF Expl. CO), Sacilerochit River, Brooks Rg. Fig. 15, right side of a conplete specimen; 16, two l e f t valves.

Plate 6