70 



OHIO FOSSILS 



Fig. 131 



lobe. A. pettiti (fig. 132) is 

 common in the Niagaran of 

 Highland County. 



Lechritrochoceras des- 



Fig. 133 



plainense (fig. 133) is coiled and each whorl of the shell is in contact with 

 the preceding one. It is common in the Guelph. 



Fig. 134 



Discoceras marshii (Lituites in older publications) is like Lechritrochoceras but the whorls 

 do not touch each other. It is common in the Guelph. 



OSTRACODA. Many species of ostracodes have been recorded from the Silurian of Ohio. 

 The one most likely to be collected, because of its size and abundance, is Leperditia angulifera 

 (fig. 134) characteristic of the Greenfield, the basal formation of the Bass Islands group, in 

 which it is abundant. These unusually large ostracodes are bean-shaped with one straight side 

 and on well-preserved specimens the eye -spot is visible. 



ARACHNOIDEA . Eurypterids (fig. 18) are scarce in our Silurian rocks but have been re- 

 ported from the Tymochtee, another formation of the Bass Islands group, near Crawford, Wyandot 

 County. They should be searched for not only for the scientific record but for the spectacular 

 quality of the fossils themselves. 



TRILOBITA. Our Silurian trilobites are surprisingly varied. They range in size from 

 tiny specimens to the huge species of Trimerus and in ornamentation from nearly smooth forms to 

 elaborately spiny and pustulose ones. Tail and head shields are the parts most commonly found 

 but an occasional complete specimen may also be expected. The key has been designed to iden- 

 tify both isolated tail and head shields and complete specimens. Only the commoner forms are 

 keyed. 



Key to the Commoner Silurian Trilobites of Ohio 

 (see p. 4 for use of keys) 



1. a) Tail shield not segmented externally, head shield smooth 



except for the eyes Bumastus 



b) Tail shield segmented externally, head shield with a 



distinct glabella 2 



2. a) With cheek spines 3 



b) Without cheek spines 5 



3. a) Tail shield pointed but not prolonged into a spine. Encrinurus 



b) Tail shield with one or more spines 4 



4. a) Tail shield with a single stout, terminal spine Dalmanites 



b) Tail shield with 3 side spines, no terminal spine Cheirurus 



