302 r.EOLor.Y of the black hills. 



tlio umbonal lidg-e and flattened on tlie lower side of the beak and anteiior 

 end, cardinal slope slightly convex and abrupt. 



Surface of the shell marked by fine lines or strijje, which are regularly 

 radiating along the umbonal ridge and on the cardinal slope, while on the 

 ventral region they strongly diverge from the umbonal line and reach the 

 ventral border at right angles to the margin. The latter striae are consid- 

 erably finer than those of the cardinal slope and do not extend to or radiate 

 from the anterior extremity of the shell, but have their origin along the 

 umbonal ridge, thereby forming an entirely different system of striae from 

 those of the cardinal slope. 



The specimens of the species which we have examined are of smaller 



size than those figured by Messrs. Meek and Hayden, and the umbonal ridge 



is more angular, while the surface of the valve below the beak for some 



distance from the anterior extremity is more compressed than shown in their 



figures. These are probably, however, only individual differences, though 



in the specimens examined are quite marked. The authors of the species 



compare it with Mytilus canceUatus Goldf. of Europe, and state that it may 



possibly prove to be identical; but specimens of that species which we have 



seen do not possess, at least do not show, the striations on the ventral side 



of the umbonal ridge as seen on those of this species. 



Formation and hcality. — In rocks of Jurassic age, near Big Horn Mount- 

 ains. 



VOLSELLA PERTENUIS. 



Plate 5, figs. 13, 14. 



Myiilvs pertemiis M. & H., Proc. Acad. Nat. Sci. Phil., March, 1858, p. 51. 

 Vohelln pertemds Meek, Siuithsoniau Check List X. A. Juia.ssic Foss., 18G4. 

 Yohclla pertemiis M. & H., Pal. Up. Missouri, p. 80, PI. 3, fig. 5. 



"Shell small, very thin and fragile, narrowly 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. Extremi- 

 ties narrowly rounded, the anal end being a little broader than the other; 

 base slightly arched behind the middle. Hinge nearly straight, and appar- 

 entl}- 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 })rojecting 



