222 



NORTH AMERICAN INDEX FOSSILS. 



91. S. leavenworthana Hall. Devonic. 

 Distinguished by its strong geniculation toward the front and by- 

 its concentric wrinkles, presenting a partial superficial resemblance 

 to Leptcena rhomboidalis. 



Helderbergian of New York. 



92. S. punctulifera (Conrad). Devonic. 

 About four fifths as long as wide. Cardinal area narrow. Striae 



strong, increasing both by bifurcation and intercalation and dis- 

 tinctly punctate. The shell outline is much more abruptly curved 

 than that of 5. headleyana. 



Helderbergian of Maine, New York, Pennsylvania, Tennessee, 

 Nevada, Quebec, New Brunswick, Cape Hilgard and Louis Napo- 

 leon of the Arctic regions. 



93. S. ampla Hall. (Fig. 269.) Devonic. 

 Large, length from two thirds to three fourths as great as width. 



Pedicle valve very concave. Brachial valve convex in the middle 

 and flat or concave at the umbo. Surface covered with angular, 

 subequal, interrupted or rugose striae which bifurcate two or three 



times before reaching the margin. In this 

 species resupination is more marked, and 

 the muscle impressions are stronger than 

 in S. headleyana. 



Onondaga of New York, Ohio, Ontario- 



94. S. reversa Hall. (Fig. 270.) 



Devonic. 

 Semi-circular to semi-elliptical, with 

 hinge line extended ; about one third of 

 the shell (young) normal, after which it 

 becomes strongly and abruptly reversed. 

 Striae simple, strong and angular near the 

 beaks, dichotomizing from one to three 

 times towards the front ; also increasing by implantation. Entire 

 surface punctate. 



In the Chemung of New York and equivalent beds of Iowa. 



Fig. 270. Strophonella 

 reversa, dorsal and ventral 

 aspects. (After Hall.) 



XXXV. Strophomena (Rafinesque) Blainville. 

 Like Rafinesquina but with the convexity of the valves reversed. 

 Pedicle valve slightly convex at umbo, becoming concave towards 

 the middle ; cardinal area conspicuous with convex deltidium ; mus- 



