JURASSIC FOSSILS 363 



beyond tlic margin. Surface marked by fine, ratlier obscure, lines of 

 grovvtli. 



"Lengtli, 0.73 incli; breadth, 0.26 incli; height, 0.30 incli." 

 The specimens of this species in the collection are internal casts in a 

 fine red sandstone, associated with Lbigulu hreciro.itris M. & H., and do 

 not show the entire external features of the shell, although the concentric 

 lines are preserved. The specimens agree pretty well with the above 

 description and figures, except that the shell is not so arcuate as represented, 

 nor so angidar along the umbonal ridge, and the anterior end is somewhat 

 narrower and less full on the buccal portion. In any of these features, 

 however, shells of this class are very liable to vary very much among the 

 diff"erent individuals of a species, as we plainly see in nearly all of the 

 living species. Messrs. Meek and Hayden compare it with Modiola minuta 

 as figured by Queenstedt (Des. Jura, Tab. 1, Fig. 3G), and observe that they 

 are at a loss to point out characters by which to distinguish it. As a 

 general rule, we do not agree with the almost indiscriminate identification 

 of species from our American formations with those of Europe, especially 

 among the non-migratorial forms of life, but with shells of this class, which 

 are fi-equently moored to floating wood or other substances, that are apt to 

 be transported to great distances without material injur}^ to the animals, 

 we see no reason why the same species might not be an inhabitant of 

 distant countries, and therefore think it even more than probable that this 

 may be identical with the above-named species of Europe. 



Formation and locality. — In fine-grained red sandstone of Jurassic age, 

 at from 80 to 100 feet above the red beds referred to the Trias, east side of 

 Spearfish Creek, near its junction with the Redwater, Black Hills. 



CRASSATELLID^. 



Genus ASTARTE Sowerby. 

 ASTARTE (?) FRAGILIS. 



Plate 5, figs. 32, 33. 



Astarte fragilis M. & H., Proc. Acad. Nat. Sci. Phil., May, 18C0, p. 183. 

 Astarte fragilis M. & H., Pal. Ui). Missouii, p. 94, PI. iv, fig. 7. 



Shell small, thin, and fragile, transversely oval in outline, nearly two- 

 thirds as high as long, with depressed convex valves, most ventricose at the 



