NATURAL SCIENCES OF PHILADELPHIA. 439 



differ from each other. In addition to this, they hold a widely different geo- 

 logical position, and are associated with an entirely different group of fossils, 

 all of which are clearly Jurassic forms. We mention these facts because Mr. 

 Marcou has expressed the opinion that the specimens in Capt. Simpson's col- 

 lections, referred by one of the authors with doubt to G. calceola. are nothing 

 more than the form he has referred to G. di/atata, an opinion he would not 

 have for a moment maintained, if he had seen the specimens alluded to. 



Locality and position. Head Wind River Valley, at base of the mountains. 

 Jurassic. 



Genus MODIOLA Lamarck.* 



MODIOLA (PEENA) FORMOSA. 



Shell narrow, subelliptical, moderately arcuate, gibbous along the umbo- 

 nal slopes, the greatest convexity being near the middle of the valves ; ante- 

 rior end rather narrowly rounded ; base sinuous near the middle, or a little 

 behind it, and rounding up rather abruptly in front ; dorsal margin a little 

 arching back to near the middle of the shell, thence becoming obtusely cari- 

 nate, and declining with a long gentle curve to the posterior extremity which 

 is very narrowly rounded ; beaks small, compressed, obtuse, and located 

 directly over the anterior margin, beyond the rounded outline of which they 

 scarcely project ; umbonal slopes prominently rounded, and extending from 

 the beaks obliquely backwards to the posterior extremity. Surface ornament- 

 ed by small concentric stria?, and a few stronger marks of growth, which are 

 crossed on the dorsal half of the valves by about forty or fifty regular, closely 

 arranged, and generally simple radiating lines, extending from the beaks 

 to the postero-dorsal and anal margins ; faint traces of another system of ex- 

 tremely fine stria? may also be seen, by the aid of a magnifier, crossing the 

 somewhat compressed ventral half of the valves, from the umbonal ridge to 

 the basal margin. * 



Length, 205 inches ; diameter from the ventral to the dorsal margins, at 

 right angles to the length, near the middle of the shell, 0-84 inch ; greatest 

 convexity at the same point, - 80 inch. 



This shell is very closely allied to the European Jurassic, Modiola cancel/ata, 

 (=Mytilus cancel/atus of Goldfuss. Petrefact. Germ., Tome ii. pi. 131, fig. 2), 

 and may possibly prove to be identical, when we can have the* means of 

 making direct comparisons with authentic specimens of the latter. The five 

 Nebraska specimens, however, now before us, all present the following differ- 

 ences from Goldfuss' figure. In the first place, they are narrower across from 

 the dorsal to the ventralside, more narrowly rounded at the posterior extrem- 

 ity, and have slightly more prominent beaks, while their antero-ventral mar- 

 gin is a little more convex, and their base rather more arcuate. Their surface 

 markings are very similar to those of Goldfuss' species, though his enlarged 

 figure represents the concentric stria? crossing the radiating lines, as being 

 more distinct and regular than on our shell. He also neither figures nor 

 mentions in his description the fine obscure transverse striae seen on the ven- 

 tral half of the Nebraska shell, though these are so indistinct that they might 

 be easily overlooked ; indeed they seem rather to be in some way dependent 

 upon the structure of the shell rather than really surface markings. 



M. Alcide D'Orbigny cites Modiola cancellata of Roemer, and his own M. 

 Strajeskiana from the Jura of Russia, as synonymous with M. cancellata of 

 Goldfuss. Without expressing a positive opinion on this point, we would re- 

 mark, that this seems very improbable to us, unless Goldfuss' species is ex- 

 ceedingly variable. At any rate, Roemer's and D'Orbigny's figures are very 

 unlike our Nebraska shell, which is remarkably uniform in its characters. 



Locality and position. Big Horn Mountains. Jurassic. 



*The name Modiola, Lamarck, will probably have to give way to Perna, Adanson 

 (non Bruguiere), or Volsella scopoli. 



1861.J . 



