342 



NORTH AMERICAN INDEX FOSSILS. 



Length 3 to 20 mm. About twenty-five species. Ordovicic- 

 Siluric. 



12. I. subnodosa Ulrich. (Fig. 1656, z-^.) Ordovicic. 

 Elongate; sides irregularly elevated or subnodose. 



Upper Trenton of Kentucky, etc. 



13. I. jonesi Wetherby. (Fig. 1656, /-w.) Ordovicic. 

 Proportionately higher than preceding, regularly convex, pos- 

 terior end more produced, hinge line with submucronate ends ; eye 

 tubercle sharp, near dorsal margin. 



Upper Trenton of Kentucky, abundant. 



14. I. cylindrica (Hall). Siluric. 

 Oval-elongate, strongly convex, so as to make conjoined valves 



nearly cylindrical. 



Medina of New York. 



Fig. 1657. a, Isochilina fabacea (15), left \alve, X ^^j ^^ Apai chites Jimbriatus 

 (18), right valve, X 10 ; c-f, Paraparchites nicklesi (19), exterior, interior and pro- 

 files of left valve ; g-i, P. humerosus (20), exterior and interior of right valve and 

 anterior view, X ^o '■> j\ ^y Dicranella bicoi'nis (40), left valve and profile, X ^o ; l-n, 

 Drepanella crassinoda (41 ), right valve, X IOj ^nd posterior view ; 0, D. elongata (42), 

 left valve, X ^^ > /» Fyurychilina reticulata (34), left valve, X ^O 5 Q~^y F- eq^o-Hs 

 (35), left valve, X 9 5 t, E. striate inarginata (36), valve, X 95 ^> ^j Macronotella 

 scofieldi (37), X ^O- (^j ^^^s'' Jones, the other after Ulrich.) 



15. I. fabacea Jones. (Fig. 1657, a.) Devonic. 



Small, elongate, regularly rounded in front, more abruptly be- 



