90 



OHIO FOSSILS 



ig. 214 



Acleistoceras ( Gomphoceras in older publications) has a 

 curved shell which is widest about the middle of the living cham- 

 ber and is narrower again at the aperture (fig. 214). Several 

 species have been recorded for the Columbus and one for the Del- 

 aware. 



Bactrites is one of the earliest ammonoids. It is small 

 and straight, looking very much like a small orthoceroid but its 

 septa have wavy edges. B_. arkonensis (fig. 215) occurs in the 

 Columbus and Plum Brook. 



Qrmoceras is also straight and has straight septal edges; 

 it is a nautiloid and much larger than Bactrites. O. winchelli 

 (fig. 216) is found in the Columbus. 



Spyroceras (fig. 217) is another straight nautiloid; its dis- 

 tinctive feature is a series of ring-like expansions and contrac- 

 tions of the shell. Two species occur in the Columbus. 



Tylorthoceras is straight and smooth except for a row of 

 tubercles on one side of the shell. T_. ohioense (fig. 218) is 

 found in the Columbus. 



Tornoceras (fig. 219) is a coiled ammonoid with narrow, 



Fig. 219 



Fig. 215 



Fig. 216 



Fig. 217 



Fig. 218 



Fig. 220 



almost closed umbilicus, very different from all our other coiled 

 cephalopods which are nautiloids. Two species occur in the Colum- 

 bus and one in the Plum Brook. 



Centroceras (fig. 220) is distinguished 

 by the row of bumps (tubercles) on the outer 

 edge of the whorl. Nassauoceras also has a 

 row of bumps but they are on top of the 

 whorl. C. ohioense occurs in the Columbus 

 and Delaware. 



Nassauoceras ( Gyroceras in older 

 publications) has a row of tubercles on top 

 of the whorl (fig. 221); see Centroceras , 

 above, for another genus with tubercles dif- 



Fig.221 



