FOSSILS OF THE JURASSIC PERIOD. 285 



factory, and, on examining specimens of that species, appears even more 

 uncertain ; still, the differences are not so great as to positively preclude 

 the possibility of specific identification. The individual represented on 

 plate 7, fig. 6, is perhaps as closely related to Meek's species as any one 

 seen, but differs very materially in the strength of the plications on the 

 lateral parts of the shell, there being from two to four on each side more 

 than on the most finely marked individuals referred to that species by its 

 author ; the shell is also less rotund and more slender and delicate in habit. 

 Genus TEREBRATULA (Llhwyd.) Brug. 

 Terebratula Augusta n. sp. 



Plate VH, figs. 7-10. 



Shell small, broadly ovate, the widest part being a little below the 

 middle of the length; width of the shell less than the length; valves 

 depressed-convex, the dorsal sometimes nearly flat, but usually two-thirds 

 as convex as the ventral ; beak small, minutely perforate, and strongly 

 incurved; cardinal slopes angular; margins of the shell acute. Surface 

 marked by lines of growth without perceptible structure except the very 

 fine punctai of the shell. 



The species of this genus, when of the same general type, are so 

 similar that it is extremely diflicult to point out specific differences or insti- 

 tute satisfactory comparisons, and the shells now under consideration belong 

 to a form which is so often repeated, both in this and several other genera, 

 that it would be useless to enter into any discussion of specific characters ; 

 therefore we shall rely upon the figures to tell their own story. 



Formation and locality. — In limestones referred to the Jurassic, at 

 Shoshone Springs, Augusta Mountains, Nevada. 



LAMELLIBRANCHIATA. 

 Genus OSTREA Linn. 



OSTREA sp. 1 



Plate VII, fifj. 12. 

 Compare 0. Engelmanni Meek, Proc. Acad. Nat. Sci. Phila., 1860, p. 311 ; Pal. Upper 

 Missouri, p. 73, figs. A and B. 

 A single example, an impression of an upper valve, found associated 

 with the following species, appears to be entirely distinct. It is a much 



