344 



NORTH AMERICAN INDEX FOSSILS. 



20. P. humerosus Ulrich and Bassler. (Fig. 1657, ^-i.) 



Carbonic-Permic. 



More convex than preceding; hinge Hne sunken from elevation 

 of dorsal portion. 



Elmdale and Wreferd formations of Kansas (Carbonic) ; Car- 

 bonic-Permic of Texas; abundant. 



IX. Primitiella Ulrich. 



Small, straight-backed, equivalved shells, usually with a broad, 

 undefined mesial depression in the dorsal slope. Genotype P. con- 

 stricta Ulrich. About twenty-five species, chiefly Ordovicic. A 

 few from the Siluric and Devonic. 



21. P. unicornis Ulrich. (Fig. 1658,;, ^, ^'.) 



Ordovicic. 



Fig. 1658. a-c; Jonesella crepidiformis (38), right, posterior and ventral views, X 

 12 ; d, e, J. pedigera (39), left valve, X '2 ; /, g, Tetradella quadrilirata (59) > two 

 right valves, X '3 *> ^» ^ ^ • Itmatifera (60), X ^4 *» /> ^> ^' ■> Pi'iniitiella unicornis 

 (21), right valve, X ^4 5 A l\ Primitia cincinnaiiemis (22), left and dorsal views, 

 X 14; ^iy fn\ n, P. tumidula (23), left valve, X '4; 0, o\ Ulrichia nodosa (29), 

 right valve, X I4> and dorsal view, X 24 ; /, /', q, U. emarginata (30), right, an- 

 terior and dorsal views, X ^4 5 ''> ^^> Halliella labiosa (31 ), left valve and dorsal view, 

 X H; -s", s' , t, Dilobella typa (33), X ^4 *» "> Ctenobolbina granosa (63), left valve, 

 X 13 ; "v, C. minima (64), right valve, X 20 ; 7v, w\ C. loculata (65), right exterior 

 and left interior, X H ; ^^ x\ Bollia unguloidea (51), left(?) valve, X '^^• (-^'^ 

 after Ulrich.) 



With a short spine near the posterior-ventral margin. 

 Trenton-Eden of the Cincinnati res:ion ; Richmond of Minnesota. 



