REPTILIAIf AGE. JURASSIC PERIOD. 107 



2. Helisoma, Swainson. 



Shell ventricose, concave on both sides ; volutions few, generally angular on one or both sides, broadly- 

 rounded on the periphery. 

 Type. — P. bicarinatus, Say. 



3. Tapbius, H. & A. Adams. 



Shell ventricose, somewhat irregular ; whorls rounded on the outer side, prominent or subangular around 

 the rather small, deeply excavated umbilical cavity of the left side. Aperture large, obovate, straight 

 within. 



Type. — P. andecolus, D'Oreisnt. 



4. Menetus, H. & A. Adams. 



Shell depressed, volutions increasing rapidly in size ; aperture suboval ; periphery more or less angular. 

 Example — P. angulatus, Phil. 



5. Anisus, FiTziNGEB. 



Shell strongly depressed ; volutions very numerous ; periphery angular. 

 Example. — P. carinatus, Mollek. 



6. Batliyomplialus, Agassiz. (= Spirorbis, Swainson, not Lamk.) 



Shell discoid, rounded on the periphery ; whorls numerous, closely embracing on the left side, exposed on 



the right ; aperture narrow, crescentic ; umbilical cavity on the left side narrow and profound. 

 Type. — Helix contorta, Linnjeus. 



'7. Gyraulus, Agassiz. (= NautiU7ia, St-eits.) 



Shell discoid, slightly convex on the right side, flat or broadly concave on the left ; volutions few, rapidly 



increasing in size, obliquely depressed, but not angular. 

 Example. — P. albus, Mijllee. 



So far as at present known, the genus Planorhis seems to have been first intro- 

 duced during the Liassic epoch ; it is also known to have been represented during 

 the deposition of the Wealden formation. Many species have likewise been fomid 

 in the fresh-water Tertiary deposits of various countries, though the genus seems 

 to attain its greatest development at the present time, and is widely distributed, 

 particularly in northern temperate regions. 



Planorbis veternns.^ a -^ ? 



(Plate IV, Fig. 1 and \a^ 

 Planotbis veternus, Meek & Hatden, Proceed. Acad. Nat. Sci. Phila. Oct. 1860, 418, 



Shell small, planorbicular ; right side distinctly concave. Volutions three and a half to four, more or less 

 rounded, increasing rather rapidly in size ; slightly concave on the inner side for the reception of each succeeding 

 whorl, and prominent or subangnlar a little within the middle on the upper side'; most convex outside of the 

 middle below. Concavity of left side rather large, basin-shaped, and, like that on the right, showing all the 

 whorls. Aperture oblique, a little oval. Surface nearly or quite smooth. 



Greatest diameter, 0.15 inch ; do. of outer whorl, 0.05 inch. 



Locality and position. — Southwest base of the Black Hills, associated with Unio 

 nucalis, Viviparus Gilli, and Valvata? scabrida; also, apparently, with Ammonites 

 Henryi. As these fresh-water shells were found in loose fragments, we are not 

 sure they hold the same position as the Ammonite, though they were picked up at 

 the base of an outcrop of hard bluish-gray limestone, in masses agreeing in their 

 lithological characters with the bed containing the Ammonite. They may possibly 

 be Tertiary species, but differ from all those we have seen from rocks of that age in 

 the Northwest. It is only provisionally we place them along with the Jurassic 

 forms. (Type No. 317.) 



