REPTILIAN AGE. JURASSIC PERIOD. 81 



characters by which it can be distinguished, than from being satisfied that it is 

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

 none of which give any idea of the form of the anterior wing ; while the_ir finer 

 surface 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 valves will be found to present characters by which this shell may be dis- 

 tinguished from Avicula Munsteri ; if so, it may take the name Pteria mucronata, 

 or Avicula mucronata, if the latter generic name is retained. 



Locality and position. — Wind River Valley, Dakota Territory. Jurassic. (Type 

 1893.) 



Genus EUMICROTIS, Meek. (Page 53.) 

 Eumicrotis curta. 



(Plate iii, Fig. 10, a, h, c, rf.) 



Avicula curta, Hall, 1852, Capt. Stansbury's Kept. Grt. Salt Lake Exp. 412, pi. 2, fig. 1, a, b. 

 Avicula (_Monotis) tenuicostata, Meek & Hayden, Proceed. Acad. Nat. Sci. Phila. March, 1858, 50. 

 Monotis curta, Meek & Hatden, Proceed. Acad. Nat. Sci. Phila. Oct. 1860, 418. 

 Eumicrolis curta. Meek, Smithsonian Check List N. Am. Jurassic Invert. Fossils, 1864. 



Shell suborbicular, very slightly oblique, usually a little higher than long, moderately convex, and nearly equi- 

 valve ; anterior side more or less rounded ; ventral and postero-ventral 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. Byssal 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 strise. > 



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 strise, 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 Monotis suhstriata, Munster, 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, Quenstedt, 

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

 suhstriata as being plano-convex, while the two valves of our sheU are nearly 

 equally convex ; andT that none of the descriptions we have read mention any difi"er- 

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

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



Locality and position. — Southwest base Black Hills. Jurassic. (Type No. 205.) 



11 September, 1864. 



