26o NORTH AMERICAN INDEX FOSSILS. 



spicuously from the umbo to the sides of the valve. Teeth promi- 

 nent. Ordovicic-Devonic. 



A. Radiating stride prominent and coarse 188. D. tesiudinaria. 



B. Radiating strioe very fine *. 



* Shell small (less than % inch wide) I. 



I. Pedicle valve wider than long a. 



a. Shell thin 189. D. emacerata. 



a. Shell convex 190. D. subaquata. 



1. Pedicle valve longer than wide 191. D. elegantula. 



* Shell more than 3^ inch wide 2. 



2. Brachial valve slightly convex and with broad mesial depression. 



193. J), subcarinata. 

 2. Brachial valve strongly convex and with slight mesial depression. 



192. D. perelegans. 



188. D. testudinaria (Dalman). (Fig. 311, a-e.) Ordovicic. 

 Pedicle valve convex at umbo, rapidly decreasing in height 



before the edge of the shell is reached ; cardinal area high ; umbo 

 but slightly incurved. Brachial valve very slightly convex or 

 depressed. Surface marked with very prominent radiating striae 

 crossed by faint concentric lines. 



Chazy-Lorraine throughout America. 



189. D. emacerata Hall. (Fig. 311,/-;;/.) Ordovicic. 

 Distinguished by its thinness, the brachial valve being flat and 



the pedicle valve depressed convex. Cardinal area almost linear. 

 Surface finely striated. 



Utica of Ohio, Missouri, Minnesota and Quebec. 



FlG. 311. a-e, Dalmanella testudinaria ; f-h, D. subicquata ; i-?/i, D. emacerata. 

 (Minnesota Geol. Surv. ) 



