94 PALEONTOLOGY OF THE UPPER MISSOURI. 



rocks, but too little is known in regard to the hinge and interior of these shells to 

 warrant the conclusion that they are beyond doubt true Astartes. Two species, 

 described by Prof. King, from the Permian rocks of England, possess the external 

 characters of this genus, and one of them, at least (A. Vallisneriana), as since 

 figiu-ed by Prof. Geinitz (Dyas. PL 12, figs. 2-4 and 25), seems also to present the 

 dentition of the Astartes. 



From the Trias, Prof. F. M. Roemer has described three species of Astarte, and 

 several are known from the Liassic rocks. In the later members of the Jurassic 

 ^stem, particidarly in the Oolites, the species are quite numerous. The genus 

 also ranges through the succeeding formations, and probably reached its greatest 

 development during deposition of the Tertiary rocks. It is, however, well repre- 

 sented in the seas of the existing epoch. The recent species are usually found in 

 nortliern latitudes. They occur on the shores of North America, Northern Europe, 

 Norway, Greenland, &.c. 



Astarte fragilis. 



(Plate iv. Fig. 7.) 

 Astarte fragilis, Meek & Hatden, Proceed. Acad. Nat. Sci. Phila. May, 1860, 183. 

 Shell small, rather broad oval, thin, moderately compressed. Anterior end rounded ; base nearly straight along 

 the middle, rounding up regularly in front, and more abruptly behind ; posterior extremity obscurely subtruncate ; 

 dorsum straight and slightly declining behind the beaks, which are small, obtuse, rather depressed, and located a 

 little in advance of the middle. Posterior umboual slopes prominent. Surface ornamented by distinct, irregular 

 concentric wrinkles, and fine parallel striae. (Hinge and interior unknown.) Pallial margins crenulate. 

 Length, 0.45 inch; height, 0.32 inch; breadth, about 0.18 inch. 



The rather unusual thinness and oval form of this little shell lead us to doubt 

 whether it can be a true Astarte, though even in this respect, as Avell as in its other 

 characters, it is evidently quite similar to some Jurassic forms generally placed in 

 that genus. 



In several respects it resembles quite closely the small thin variety of A. excavata, 

 Sowerby sp., figured by Morris and Lycett in their monograph of the English Oolitic 

 fossils, pi. 9, fig. 19 (Palaeont. Soc), but its beaks are located further back, and its 

 posterior margin is narrower and more oblique ; while its luuule does not seem to 

 be excavated as in that species. 



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

 beds of that region. (No. 194.) 



Astarte inornata. 



(Plate III, Fig. 12, a, I.) 

 Astarte inornata, Meek & Hatden, Proceed. Acad. Nat. Sci. Phila. May, 1860, 183. 

 Shell subelliptical, compressed ; extremities rounded, the posterior margin forming a broader curve than the 

 other. Base semi-elliptical in outline ; dorsum declining from the beaks, the anterior slope being a little concave, 

 and the other nearly straight, or slightly convex. Beaks moderately elevated, compressed, located just in 

 advance of the middle. Lunule rather deep, lance-oval, bounded on each side by a more or less distinct angle. 

 Surface marked by concentric strine, with a tendency to develop small, very obscure concentric wrinkles. 

 Length, 1.15 inch; height, 0.79 inch; breadth, about 0.44 inch. 



We only place this shell in the genus Astarte from its similarity to several 

 Jurassic species of that group, not having seen its hinge or interior. 

 Locality and jiositioii. — Same as last. (No. 202.) 



