REPTILIAN AGE. JURASSIC PERIOD. H3 



If the following des( ribcd species really belongs to this genus, it would seem to 

 establish tlie existence ol' tlie group during the deposit ion of tlie middle or older 

 members of the June-ir M-inn. This conclusion, IK. \ve\er, should not be adopted 

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

 graphical position. Tlie existence of this genus during the "Wealdcn 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). 

 Yalvata? scabrida. 



(Pun IV, Fig. 2, a, &.) 

 Valvatat icabrida, MEEK & HAYDEX, Proceed. Acid. Nat. Sol. Phila. Oct. .860, 418. 



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

 pi/-, very convex ; subangular around the upper outer aide, and horizontally flattened between the angle and the 

 snturn; vmtrifoe 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 0.16 inch. 



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

 hard, brittle, argillo-calcarcous 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 hi 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 

 r< it rota, 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. 



Lo<-n Hi ;i and position. Near southwest base Black Hills, where it was found hi 

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

 orbis retermi* and Unio nucalis. These masses were found at the base of an outcrop 

 of very similar rock, containing Ammonites Henryi, 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, arc probably of Jurassic ages. 

 (Type, 316.) 



FAMILY VIVIPARID^E. 



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

 thin, or more or leas 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 



16 January, 1865. 



