ORDOVICIAN FOSSILS 



51 



mens in our museums. Only the commoner species are discussed here. For the others, see 

 Ulrich (1895, pp. 627-693). 



Pelecypods are often preserved as internal molds with closed or open valves. These do 

 not show the external characters but may be quite useful for identification and should be col- 

 lected. The commoner genera can be recognized by external characters but more certain 

 identification can be made if the interior of the shell is available for study. 



Key to the Commoner Ordovician Pelecypods of Ohio 

 ( see p. 4 for use of keys ) 



1. a) Shell with strong radiating ribs and weak concentric ornamentation 2 



b) Shell without radiating ribs 3 



2. a) Beaks terminal, shell winged Opisthoptera 



b) Beaks almost terminal, shell not winged Byssonychia and Anomalodonta 



3. a) Shell winged Pterinea 



b) Shell not winged 4 



4. a) Shell longer than high, beak small 5 



b) Shell almost as high as long, beaks large Cyrtodontula 



5. a) Shell with an internal diagonal ridge Ischyrodonta 



b) Shell without internal diagonal ridge Modiolopsis 



The following genera have strong radiating ribs and weak 

 concentric ornamentation. In the genus Byssonychia the beaks 

 are almost terminal, that is they are near the apex of the tri- 

 angle formed by the shell. B. radiata (fig. 68) is about 2 

 inches high and 1. 5 inches long; it is found in all Cincinnatian 

 formations. B. grandis is much larger than P. radiata (2 

 inches long and 2. 5 inches high) and is also more convex, i. e. 

 thicker through; it is found from the Waynesville to the White- 

 water. Anomalodonta gigantea is like Byssonychia but 

 larger and with broader ribs; it also differs from 

 Byssonyc hia internally; an average specimen is 3 l/8 

 inches high and 2 3/4 inches long; it occurs in the 

 Richmond. Opisthoptera differs from the preceding 

 two genera in being longer than wide; is has ter- 



Fig. 69 



Fig. 68 



minal beaks, strong radiating ribs, and a large 

 wing (that is, a pointed part of the shell on one 

 end of the hinge line) at the end of the shell 

 away from the beak; there are several species, 

 all from the Richmond; an average specimen is 

 3 inches long and 2 inches high. 



The remaining genera have no radiating 

 ribs. In Pterinea demissa (fig. 69) the shell 

 has two wings, one on each side of the beak, 

 which is almost in the center of the hinge line; 



