52 



OHIO FOSSILS 



the species has distinct concentric ornamentation; an average 

 specimen is 2 inches long and 2 inches high; it is found in all 

 Cincinnatian formations. Modiolopsis modiolaris (fig. 70) is 

 longer than high (1. 75 inches long, 7/8 inch high) and the shell 

 is narrowest in the region of the beak; it lacks the diagonal ridge 

 of the next genus, Ischyrodonta ; the beaks are inconspicuous; it 

 is found in all Cincinnatian formations. Ischyrodonta elongata 

 is like Modiolopsis but with a diagonal ridge running from the 

 beak to the lower margin; specimens are about 2. 25 inches long 

 and 1. 5 inches high; it occurs in the Whitewater, gyrtodontula 

 umbonata is large, about 2. 25 inches long and 1. 75 inches high, has large anterior but not ter- 

 minal beaks and is thicker through than either Modiolopsis or Ischyrodonta ; it is found in the 

 Whitewater formation. This species is called Whitella umbonata in older publications. 



GASTROPODA . Several kinds of gastropods are found in our Ordovician. They are never 

 as abundant as the brachiopods and bryozoans. All of them are small (an inch or less long) and 

 their preservation is often poor. Only the commoner species are described here. 



Fig. 71 



Cyclonema bilix (fig. 71) looks like a top slightly 

 askew, with many fine thread-like spiral markings 

 and weaker axial ornamentation (i. e. parallel to 

 the long dimension of the shell); the species oc- 

 curs throughout the Richmond. 



Cyclora minuta and C. parvula are like 

 Cyclonema bilix but much smaller; they are abun- 

 dant in some beds of the Cincinnatian. 



Ftg. 72 



In Lophospira bowdeni (fig. 72) the spire is much longer than in Cyclonema 

 and Cyclora and the whorls have a sharp keel which is characteristic; the species 

 occurs in all Cincinnatian formations. 



In Sinuites cancellatus (fig. 73) the earlier 

 whorls of the spire are entirely hidden by the last 

 whorl; the species is found in all Cincinnatian for- 

 mations. 



Fig. 73 



CEPHALOPODA. Straight-shelled nautiloid cephalopods are common in the Ordovician 

 of Ohio. Some nautiloids with curved shells have also been found. The spirally coiled forms 

 are rather rare. Cephalopods are not readily identifiable from the external characters. Gen- 

 era are based on the character of the siphuncle and its structure where it goes through the 

 septa. For this reason natural sections, that is specimens in which half the shell has been 

 worn away, showing the siphuncle, should be collected. They are not uncommon in our Ordo- 

 vician. Only three species of the many found in our rocks are common enough to be mentioned 

 here. 



Endoceras proteiforme (fig. 74) has a straight shell with a siphuncle that is very large, 

 as much as half the diameter of the shell; it is found in all Cincinnatian formations. Ortho- 

 ceras dyeri is like Endoceras proteiforme but has a proportionately smaller siphuncle; it oc- 

 curs in the Corryville member of the McMillan formation. Other species of Orthoceras and 

 closely allied genera are found throughout the Cincinnatian. Gyroceras baeri (fig. 75) is one 

 of our spirally coiled forms; an average specimen is about 4 inches across; it is found in the 

 Liberty formation. 



