RE PTI LI AX AGE. J UR A SSI C PERIOD. 107 



2. llfli-om:t. 



Shell vcntricoHe, concave on both sides ; volutions few, generally angular on one or both Bides, broadly 



roiiM.i.-.l on the pfriplirry. 

 Ty/it. I', bicarinalut, SAT. 



3. Tupliius, II. & A. AnAM.t. 



Shell ventrioose, somewhat irregular ; whorls rounded on the outer side, prominent or snbangular around 

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

 will, in. 



Tyfit.l'. andecolut, D'Oanio.xT. 



4. MrnrtiiH, II. & A. ADAMS. 



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

 Example /'. amjulatua, PUIL. 



5. Antstis, FiTZiitaiB. 



Shell strongly depressed ; volutions very numerous ; periphery angular. 



/ c,im}'le. /'. carinatut, MrLLER. 



. llatliyniupliiilus AOAS.XIZ. (= Spirorbit, SWAIKBOK, not LAHK.) 



H i.-ll discoid, rounded on the periphery ; whorls numerous, closely nmbraoing on the left side, exposed on 



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

 Type Helix contorta, LlRKACS. 



1. Gyraultis, AOABRIX. (= Nautilina, STBIH.) 



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



increasing in size, obliquely depressed, bat not angular. 

 Example. P. albui, MCLLEB. 



So far as at present known, the genus Planorbis 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 found 

 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 veternus. 



(Put* IV, Fig. 1 and la, b.) 

 Ptanorbi* veternus, MEEK & HATDKX, Proceed. Aoad. Nat. Sol. Phila. Oct. I860, 418. 



Shell small, planorbioular ; right side distinctly concave. Volutions three and a half to fonr, 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 snbangnlar a little within the middle on the npper 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 

 nucalix, Vir!ji<irn# Gi/ti, and Valvata? sedbrida; also, apparently, with AmmonHts 

 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.) 





