SILURIAN FOSSILS 



67 



3. 

 4. 

 5. 

 6. 

 7. 

 8. 

 9. 



Key to the Commoner Silurian Gastropods of Ohio 



( see p. 4 for use of keys ) 



Height of shell greater than its width 2 



Height and width of shell almost equal 3 



Height of shell less than its width 6 



Whorls strongly rounded, giving the spire the 



appearance of a string of beads Hormotoma 



Whorls nearly flat -sided Coelocaulus 



Aperture longer than the spire 4 



Aperture shorter than the spire 5 



Axial ornamentation frill-like Phanerotrema 



Axial ornamentation thread-like Platyostoma 



Whorl with strong upper, middle, and lower angulations or keels. . . Trochonema 

 Whorl without keels, angulated on base only Cyclone ma 



Shell coiled in one plane or almost 7 



Shell not coiled in one plane 9 



Inner edge of aperture free from spire Platyceras 



Inner edge of aperture not free from spire 8 



Whorls angulated above and below Poleumita 



Whorls rounded, not angulated Tremanotus 



Umbilicus wide . „ Straparolus 



Without umbilicus Strophostylus 



Fig. 117 



Of the two long-spired gastropods in our Silurian, Hormotoma is 

 the commoner. It can be distinguished from Coelocaulus by its much 

 more rounded whorls which give specimens the appearance of a group 

 of beads arranged according to size. Even in inter- 

 nal molds, this character is apparent and distinc- 

 tive. Hormotoma whiteavesi (fig. 117) is common 

 in the Guelph. 



Coelocaulus (fig. 118) also has a long spire 

 but its whorls are flat -sided; in internal molds, they 

 are squarish in section, which distinguishes them 

 from Hormotoma . Coelocaulus macrospira is com- 

 mon in the Guelph. 



In the following four genera, the height and 

 width of the shell are more or less equal. In the 

 first two, the aperture is longer than the spire 

 and the shell is therefore somewhat globular. 

 Phanerotrema has characteristic axial ornamen- 

 tation of wavy, frill-like threads. P. occidens 

 (fig. 119) is common in the Springfield and Cedarville. 



Platyostoma ( Diaphorostoma in some older 

 publications) has axial ornamentation also, but it is 

 straight, not wavy. P. cornutum (fig. 120) is abun- 

 dant in the Lower Niagaran. 



Fig. 118 



