68 



OHIO FOSSILS 



In the last two genera of this group, 

 the aperture is shorter than the spire and 

 the shell is rather top-like. Trochonema 

 can be recognized immediately by the 

 sharp angulations on the top, middle, and 

 bottom of each whorl. The angulations 

 are so sharp in some species that they 

 can be called keels. T. fatuum (fig. 121) 

 is common in the Cedarville and Guelph. 



Fig. 120 



Fig. 119 



Cyclonema (see fig. 71) lacks the angulations of Trochonema 

 except for a rather faint one on the base. Its main feature is a 

 series of small but distinct spiral threads cut by fainter axial ones. 

 Cyclonema ohioensis is common in the Springfield.. 



The remainder of the genera figured are wider than high. The 

 following three genera are coiled in one plane or almost. Platyceras 

 has few whorls which expand rapidly; its most dis- 

 tinctive feature is that the edge of the last whorl is 

 NV free. Platyceras angulatum (fig. 122) is common 

 in the Niagaran. 



Poleumita is angulated above and below. The 

 shell surface has 2 kinds of ornamentation; the ax- 

 ial ornamentation is frilled on the upper surface, 

 straight and thread-like below. P. scamnata (fig. 

 123) is rare in the Cedarville and common in the Guelph. 



Tremanotus is one of our spectacular genera. It has a widely flaring aperture like a bu- 

 gle. Even if the aperture is not preserved, the strong and numerous spiral threads will distin- 

 guish this genus from other Silurian ones. Internal molds have small bumps in a line on the 

 outer edge of the whorl; these correspond to a series of holes in the shell like 

 those in the modern abalone. Tremanotus alpheus (fig. 124) is common in the 

 Guelph. 



The last two genera are not coiled in one plane. 

 Straparolus (fig. 125) is shaped like a low cone; the 

 whorls are round in section and the umbilicus is very 

 wide. Straparolus paveyi is common in the Niagaran. 



In Strophostylus the shell is globose and there is no umbilicus. 

 . cancellatus (fig. 126) is common in the Niagaran. 



Fig. 125 



V \ \ \ l - 



CEPHALOPODA. Straight, curved, and coiled 

 cephalopods are fairly common in our Silurian. Some 

 of the straight forms reach large size (2 feet long in 

 one incomplete shell in Orton Hall) and some of the 

 curved forms have a narrowed aperture, sometimes 

 a mere slit in the middle of the 

 shell, ending in two lobes on 

 the sides. This gives internal 

 molds the appearance of small 

 sacks of salt or cement. The 

 following key will help -distin- 

 guish the genera and some 

 species. 



Fig. 126 



Fig. 124 



