5i» 



NORTH AMERICAN INDEX FOSSILS. 



and in the presence of a striated ligament area. It also lacks the 



mesial depression of Modiolopsis. Ordovicic. 



424. E. plana Hall. (Fig. 696, a, b.) Ordovicic. 



Small, triangular, marked with strong concentric growth lines. 

 Type of genus. 



Stones River of Minnesota, and Wisconsin (Lower Blue lime- 

 stone). 



CXIV. Aristerella Ulrich. 



Subovate, small, moderately convex, nearly smooth, inequivalve. 

 Left valve the smaller. No mesial furrow present. Hinge appar- 

 ently very thin and without teeth. Muscle and pallial impressions 



b ~ c 



Fig. 696. a, b, Eurymya plana, interior of left valve and its hinge enlarged ; c-e y 

 Aristerella nitidula, natural size and enlarged. (Minn. Survey.) 



as in Whiteavcsia. Differs from Eurymya and Whiteavesia in its- 

 unequal valves. Ordovicic. 

 425. A. nitidula Ulrich. (Fig. 696, c, d> e.) Ordovicic. 



Minute ; surface nearly smooth. Type of genus. 



Black River : Minnesota. 



CXV. Goniophora Phillips. 



Equivalve, very inequilateral, obliquely truncate posteriorly,. 



rounded anteriorly. Cardinal line straight. Beaks small and 



closely incurved. Umbo prominent. A strong angular ridge 



extends from the umbo to the posterior margin and a broad, un- 



FlG. 697. Goniophora ciubia, right valve enlarged and natural size. Cardinal view 

 of a complete specimen, natural size. (After Whitfield.) 



defined sinus from umbo to base. Surface marked by concentric 

 striae. Ligament external. Like Modiomorpha in internal charac- 



