SILURIAN FOSSILS 



69 



1. a 

 b 



2. a 

 b 

 c 



3. a 

 b 



4. a 

 b 



5. a 

 b 



Key to the Commoner Silurian Cephalopods of Ohio 



( see p. 4 for use of keys ) 



Shell straight 2 



Shell curved 4 



Shell coiled 5 



Shell with fine longitudinal striae only Orthoceras simulator 



Shell with concentric striae only Dawsonoceras 



Shell with longitudinal and concentric striae : 3 



Striae fine and close together Protokionoceras 



Striae fine but with wider spaces between them Kionoceras 



Aperture two-lobed Phragmoceras 



Aperture with only one lobe Amphicyrtoceras 



Whorls touching Lechritrochoceras 



Whorls not touching Discoceras 



Orthoceras simulator (fig. 127) has a straight shell with fine longitu- 

 dinal striae only. Externally it resembles the next genus, Dawsonoceras 

 which has only concentric striae, and Protokionoceras and 

 Kionoceras which have both concentric and longitudinal striae. 

 Identification in these genera will be possible only when the 

 external ornamentation is preserved; internally they look 

 much alike except for the characters of the siphuncle which 

 are visible in sections. 



Dawsonoceras annulatum (fig. 128) is easily recognized 

 by the straight shell with concentric striae only. Even when 

 the exterior is not preserved, the species can be recognized 

 because of the ring-like expansions of the shell. The species 

 is rare in the Dayton, common in the Springfield and Guelph. 



Protokionoceras (fig. 129) has a straight shell 

 with fine longitudinal and concentric striae which are 

 close together, a feature which distinguishes it from 

 Kionoceras in which they are farther apart. Both our 

 species are called Orthoceras in older publications. 

 They are common in the Cedarville and rare in the 

 Guelph. 



Kionoceras (fig. 130) is much like Protokionoceras 

 but its longitudinal and concentric striae are fewer and 

 farther apart. Three species are common in the Niagaran. 



Fig. 127 



Fig. 128. 



Phragmoceras has a curved shell, narrowed at the 

 top; the aperture is narrow in the center with two nearly 

 round expansions at the sides. When the aperture is not 

 preserved, it can be mistaken for Amphicyrtoceras . P. 

 parvum (fig. 131) is common in the Cedarville and rare 

 in the Guelph. 



Amphicyrtoceras ( Qncoceras in older publications) 

 is like Phragmoceras but the narrow aperture has only one 



Fig. 128 



