33^ 



NORTH AMERICAN INDEX FOSSILS. 



37c. S. increbescens Hall. (Fig. 425, c-f.) Mississippic. 



Gibbous. Hinge line always as long as the greatest width of 

 the shell and terminating in more or less extended mucronate tips 

 which are often unequal on the two sides of the shell. Whole 

 surface covered with plications, those on the fold and sinus extend- 

 ing only part way to the umbo and narrower than those on the 

 sides of the shell. 



Kaskaskia of Illinois and Kentucky. 



Fig. 427. Spirifer logani, X %• (After Hall.) 



Fig. 428. Spirifer leidyi. (After Whitfield. ) 



371. S. striatus (Martin). Carbonic. 

 Pedicle valve the more convex and marked by a broad, ill de- 

 fined mesial depression ; beak small, pointed and closely incurved. 

 Entire surface of valves marked by plications which are nearly 

 uniform in size with little tendency to become fasciculate as in 

 5". cameratus. 



Utah, Nevada, New Mexico, Nova Scotia. 



372. S. cameratus Morton. (Fig. 429.) Carbonic, 

 Of medium size or large, broadest at hinge line, with cardinal 



extremities often pointed. Pedicle valve with concave cardinal 

 area of moderate height. Mesial fold and sinus as well as the rest 

 of the shell covered with many striae of unequal size, usually 

 arranged in bundles (fasciculate). 

 Throughout North America. 



