226 



NORTH AMERICAN INDEX FOSSILS. 



is often abruptly and rectangularly deflected. The whole exterior 

 is marked with fine radial striations. Ordovicic-Carbonic. 

 101. L. rhomb oidalis (Wilckens). (Fig. 273, a; 273, b.) 



Ordovicic-Lower Carbonic. 



Small, usually semicircular; cardinal extremities often much ex- 

 tended. Pedicle valve slightly convex near the hinge, slightly con- 

 cave just before the deflection. 



Trenton-Waverly ; generally distributed throughout America 

 and Europe. 



J*J * a A 



Fig. 273, b. Leptcena rhomboidalis, interior of brachial valve, profile and dorsal 

 views. Helderbergian type. (After Hall & Clarke.) 



102. L. unicostata Meek and Worthen. (Fig. 271, i-j.) 



Ordovicic. 



Distinguished from the preceding by the complete absence of 

 concentric wrinkles and the presence of a small mesial ridge on the 

 outside of the pedicle valve. 



Lorraine of Illinois, Wisconsin, Minnesota, Iowa, Manitoba. 



XXXVII. Plectambonites Pander. 

 Small. Pedicle valve convex, brachial concave. Hinge line 

 forming greatest width of shell. Cardinal areas narrow. Car- 

 dinal process large and simple, almost filling the delthyrium. 

 Dental plates continued around the long, narrow, muscular areas. 

 Surface striae very fine, often alternating in size. Ordovicic- 

 Siluric. 



