i: KPTILIAN AGE. .ir I: A SSIC PERIOD. 81 



character^ by which it can he distinguished, than from being satisfied that it is 

 really identical. The tew specimens in the collection consist of imperfect left valves, 

 none of which gi\e any idea of the form of the anterior wing ; while their liner 

 surtace markings are nearly obliterated by exfoliation and weathering. It is more 

 than probable that perfect specimens showing the nature of the surface markings 

 of both \alves will be found to present characters by which this shell may be dis- 

 tingnished from .\<-;<-nl Mmixf,,-i ; if so, it may take the name Pteria mucroiiata, 

 or Aririi/u linn i-i'iKilii, if the latter generic name is retained. 



I.<-nIir>/ n/nl jH^iridii. Wind Itivcr Valley, Dakota Territory. Jurassic. (Type 



Genus EUMICROTIS, MKKK. (Page o:j.) 



I IIIIIM roli- Clirtn. 



(PLATE iil, Fig. 10, a, I:, C, rf.) 



Avieula curia, HAIL, 1852, Capt. Sunsbnry's Kept. Grt. Salt Lake Ezp. 412, pi. 2, fig. 1, a, b. 

 .InVu/fi (JJomalit) tmuicottata, MEEK & HAYDHX, Proceed. Aead. Nat. Bel. Pliila. March, 1868, 60. 

 .M-<mtit curia, MKKK & HATDES, Proceed. Aoad. Nat. Sci. Phila. Oct. 1860, 418. 

 Kumicrotis curia, MEEK, Smithsonian Check LUt N. Am. Jurassic Invert. Fossils, 1864. 



Bhvll saborbicular, very slightly oblique, usually a little higher than long, moderately convex, and nearly eqnl- 

 valve ; anterior side more or lr*s rounded ; ventral and postero-vi-ntr.il margins rounded. Posterior border ascend- 

 ing obliquely forward, and sometimes slightly concave in outline above. Hinge margin straight, and considerably 

 shorter than the length of the shell, compressed behind, so as to form a very short, more or less angular wing, 

 which is sometimes nearly obsolete ; rounding or very obtusely angular In front of the beaks, but not terminating in 

 a wing. Beak of left valve small, and rising a little above the hinge ; that of the other valve more compressed, 

 and scarcely distinct from the cardinal margin both located slightly in advance of the middle. Bysnal sinus 

 small, rather deep, angular, and connected with a narrow external groove extending nearly parallel with the hinge 

 margin to the beak. Surface of left valve ornamented by radiating lines ; that of the right valve generally only 

 marked with concentric strin. 



Length, 0.60 Inch ; height, 0.64 Inch ; breadth or convexity, 0.26 inch. 



The radiating lines of the left valve are regular, and usually rather smaller than 

 the depression between ; they seem to be always simple, though many of them die 

 out before reaching the beaks. On the right, or smaller valve, they are always 

 very obscure, and often obsolete, while the concentric stria*, in most cases, are 

 moderately distinct. In adult shells, the hinge line is often proportionally shorter 

 than in smaller individuals. 



This species varies in form, some of the specimens being longer, and some shorter 

 than wide. Those figured by Prof. Hall are in a bad state of preservation, and 

 give an imperfect idea of the characters of the species; though we are satisfied, 

 from direct comparison with other specimens obtained at the same locality, as well 

 as with those collected by Capt. Stansbury, that our shell belongs to this species. 



In form, surface markings, and indeed in almost all its characters, this shell 

 agrees so very closely with Moiiotis sulmtriata, Munstcr, that we are strongly in- 

 clined to the opinion that it will, on comparison, prove to be identical. It is gene- 

 rally a little less oblique than the figures of that shell given by Goldfuss, Quenstcdt, 

 and others, but varies in this respect. Were it not that some authors describe M. 

 Kiilistrinta as being plano-convex, while the two valves of our shell are nearly 

 equally convex ; and that none of the descriptions we have read mention any differ- 

 ence in the distinctness of the radiating stria? on the two valves, we would scarcely 

 hesitate to refer the specimens now before us to M. mlbttriafa. 



Locality und ponltion. Southwest base Black Hills. Jurassic. (Type No. 205.) 



11 September, 18O4. 



