112 



OHIO FOSSILS 



Fig. 306 



The shell in Marginifera (fig. 306) is comparatively small. The length 

 is about 2/3 of the width and one valve is strongly convex, the other almost 

 flat. Two species are common. 



Linoproductus has fine, numerous radiating ridges which appear to be 

 free of spines. Close examination will show the broken bases of a few 



scattered, slender spines on their surface. Our species 

 of Linoproductus can be distinguished from the other 

 productids by the near absence of concentric ornamen- 

 tation of the shell. L. cora (fig. 307) is the abundant 

 species. 



In the next two genera, the concentric and radial 

 ornamentation are about equally developed. In 

 Juresania (fig. 308) they form wart-like bumps regular- 

 ly arranged on the surface of the shell. Three species, 

 two of them common, are found in our 

 Pennsylvanian. 



Dictyoclostus resembles 

 Juresania but its~radial and concentric 

 ornamentations are continuous in- 

 stead of separated into bumps. This gives the surface of the shell the ap- 

 pearance of a network of square pits, most distinct in the beak region, less 

 so away from the beak. p. semireticulatus (fig. 309) is com- 

 mon. 



PELECYPODA . The marine limestones and shales of the 

 Pennsylvanian contain an abundance of pelecypods. Twenty -two 

 genera are common enough to be noted here. The brackish or 

 fresh-water shales interbedded with the coals of Ohio contain, in 

 places, great numbers of pelecypods of peculiar character. They 

 belong to the genus Naiadites and may be the first truly non- 

 marine pelecypods. The following key will help identify the 

 commoner forms. 



Fig. 309 



Key to the Commoner Pennsylvanian Pelecypods of Ohio 

 (see p. 4 for use of keys) 



1. a) Shell with one or two distinct "wings" on the hinge 2 



b) Shell without "wings" 12 



2. a) Shell with one wing only 3 



b) Shell with 2 distinct wings 4 



3. a) Length more than twice the height Aviculopinna 



b) Length and height nearly equal Posidonia 



4. a) Beaks at or near one end of the shell 5 



b) Beaks near the center of the shell 6 



