REPTILIAN AGE. JURASSIC PERIOD. 113 



If the following described species really belongs to this genus, it would seem to 

 establish the existence of the group during the deposition of the middle or older 

 members of the Jurassic system. This conclusion, however, should not be adopted 

 until all doubts are removed in regard to its generic characters, and exact strati- 

 graphical position. The existence of this genus during the Wealden period is, 

 however, well established. It is also well represented in the fresh-water Tertiary 

 deposits of Europe and occurs in the Tertiary beds of the central districts of North 

 America, and probably attains its greatest development at the present time. 



Subgenus TROPIDINA, H. & A. Adams (p. 112). 

 Valvata? scabrida. 



(Plate IV, Fig. 2, a, b.) 

 Valvata? scabrida, Meek & Hayden, Proceed. Aoad. Nat. Sci. Phila. Oct. ^860, 418. 



Shell small, subglobose, tliiu ; spire rather depressed. Volutions three to three and a half, increasing rapidly in 

 size, very convex ; subangular around the upper outer side, and horizontally flattened between the angle and the 

 suture ; ventricose on the outer and under sides. Suture well defined ; aperture round, oval ? Surface ornamented 

 by strong, regular marks of growth. 



Length and breadth, each about O.IG inch. 



Not having seen specimens of this little shell sufficiently detached from the very 

 hard, brittle, argillo-calcareous matrix, to show very satisfactorily the form of its 

 aperture, we are left in some doubt respecting its generic relations. From its 

 general appearance, however, taken in connection with the fact that a few other 

 fresh-water shells occur in the same bed, we are led to regard it as most probably a 

 Valvata, though it may possibly be a Viviparus, or belong to some marine genus. 

 Its principal distinguishing characters are the small number and ventricose form of 

 its whorls, and strongly defined marks of growth. 



Locality and position. — Near southwest base Black Hills, where it was found iu 

 loose fragments of a hard bluish-gray argillo-calcareous rock, associated with Plan- 

 orhis veternus and Unio nucalis. These masses were found at the base of an outcrop 

 of very similar rock, containing Ammonites Henryl, and seem to belong to the same 

 formation. The fresh-water species may be Tertiary, though we think they and the 

 Ammonite, with which they appear to be associated, are probably of Jurassic ages. 

 (Type, 316.) 



Family VIVIPAEID^. 



Shell varying from subglobose to turbinate, or conical subovate ; rather 

 thin, or more or less thickened, covered with an epidermis ; surface 

 smooth, spirally striate, or with revolving, rarely nodose, carina? ; aper- 

 ture oval or subcircular, simple, and rounded anteriorly ; peritreme con- 

 tinuous, simple. 



Operculum annular, or rarely with a subspiral nucleus. 



Animal retractile within the shell ; foot moderate, and adapted for 

 crawling only ; rostrum moderate, nearly or quite entire at the extremity ; 

 tentacles tapering, retractile, and having the eyes on short tubercular 



15 January, 1865. 



