86 PALEONTOLOGY OF THE UPPER MISSOURI. 



also occurs in the Cretaceous and Tertiary deposits, and is abundantly represented 

 in our existing seas, where it probably attains its maximum development. The 

 recent species are chiefly found in southern latitudes, though a few occur on the 

 coast of Great Britain, and in the INIediterranean and Arctic seas ; also on the east- 

 cm coast of the United States, &c. 



Yoli^ella pei'tenui«. 



(Plate III, Fig. 5, 5a.) 

 MijUlus pertenuis, Meek & Hatden, Proceed. Acad. Nat. Sci. Phila. March, 1858, 51. 



Comp. Modiola miniila, Quesstedt, Der Jura, tab. i, fig. 36. 

 Volsella pertenuis, Meek, Smithsonian Check List N. Am. Jurass. Fosa. 1864, 28. 



Shell small, very thin and fragile, narrow oblong-oval, slightly arcuate ; valves convex along the middle, from 

 near the beaks obliquely backward, in the direction of the lower part of the posterior end. Extremities narrowly 

 rounded, the anal end being a little broader than the other ; base slightly arched behind the middle. Hinge nearly 

 straight, and apparently rather less than half the length of the shell ; dorsal margin forming a broad descending 

 curve from the back extremity of the hinge posteriorly. Beaks small, subangular, and located at the anterior end, 

 scarcely projecting beyond the margin. Surface marked by fine, rather obscure, lines of growth. 



Length, 0.73 inch; breadth, 0.26 inch; height, 0.30 inch. 



This shell is so very similar to a Jurassic species figured by Quenstedt (Der Jura, 

 tab. i, fig. 36), under the name of Modiola minuia, that, after further comparisons, 

 Ave are at a loss to point out any characters by which it can be distinguished. As 

 there seems, however, to be some reason to doubt the identity of the shell figured 

 by Quenstedt with M. minuta of other authors, we have concluded to retain our 

 name, jjertennls, until authentic specimens of these shells can be compared. 



The species now under consideration is also similar in form to yomig specimens 

 of MytUus Meekii, Evans & Shumard (Trans. St. Louis Acad. Sci. vol. i, p. 40), but 

 is shorter in proportion to its height. The fact, too, that M. Meehii is an upper 

 Cretaceous species, while that now before us occurs in rocks holding a rather low 

 position in the Jurassic system, is conclusive evidence, we think, that they must 

 difi'er specifically. 



If Adanson's ante-Linnsean genera are to be adopted, with his first species of each 

 as its type, the name of our shell would have to be written Perna pertenuis, as it 

 belongs to the same group as the tj^oe of Perna, of that author (not of Brug., Oken, 

 or Cuv.). If, on the contrary, neither Perna, Adanson, nor VolseUa, Scopoli, should 

 be retained, we must adopt Modiolus, Lamarck, and call it Modiolus 2^'^>'fenuis- 



Locality and position. — Southwest base of the Black HiUs, in the lower Jurassic 

 of that region. (Type 215.) 



Tolsella rormosa. 



Modiola (Perna) formosa, Meek & Hayden, Proceed. Acad. Nat. Sci. Phila. Dec. 1861, 439. 

 Volsella formosa, Meek, Smithsonian Check List N. Am. Jurass. Fossils, 1864, 28. 



Shell elongate-subelliptical, a little arcuate, gibbous along the oblique umboual slopes, from the beaks to the 

 postero-basal extremity ; greatest convexity near the middle of the valves. Ventral margin somewhat sinuous 

 near the middle, or a little behind it, and rounding up rather abruptly at the extremities ; dorsal outline broadly 

 arcuate, declining from the middle posteriorly ; both extremities rather narrowly rounded. Beaks small, some- 

 what compressed, obtuse, and located directly over the anterior margin, beyond the outline of which they scarcely 

 project. Surface ornamented with small concentric stria;, and a few stronger marks of growth, which are crossed 

 on the dorsal and postero-dorsal regions, by regular, closely arranged, and generally simple radiating lines. Faint 

 traces of another system of extremely fine stria; may be also sometimes seen by the aid of a magnifier, crossing 

 the somewhat compressed ventral region of the valves, from the oblique umbonal ridge, nearly at right angles to 

 basal margin. 



