3o8 



NORTH AMERICAN INDEX FOSSILS. 



308. Z. modesta Hall. (Fig. 385,/-/.) Ordovicic. 

 Width slightly exceeding length. Hinge line somewhat ex- 

 tended. Pedicle valve convex with a median ridge occupied by 

 four stronger plications; beak prominent and incurved. Brachial 

 valve depressed-convex and nearly circular with an ill defined 

 mesial sinus. Plications simple, about 18. 



Utica to Richmond of New York, Ohio, Missouri, Wisconsin* 

 Iowa, Minnesota, Ontario. Clinton of Ohio. 



308^. var. cincinnatiensis James. Ordovicic. 



Differs from the preceding in its larger size, more pronounced 

 median fold and sinus, and coarser, more angular bifurcating pli- 

 cations. 



Cincinnatian of Ohio, Tennessee, etc. 



309. Z. nicoletti Winchell and Schuchert. (Fig. 386, a-c.) 



Ordovicic. 



Very small, usually not much exceeding yi inch in width. 



Length greater than breadth. In these respects it resembles Z. 



recunnrostris ; it differs from that species in the faintness of its 



Fig. 386. a-c, Zygospira nicoletti, three views much enlarged ; d-h, Cyclospira 

 bisulcata four views enlarged X 2 > and brachidium still further enlarged X 4- (After 

 Winchell and Schuchert.) 



plications and in the presence of a sinus on the pedicle valve and 

 a fold on the brachial, being in these respects exactly opposite. 

 Trenton of Minnesota, Wisconsin and Missouri. 



