SILURIAN FOSSILS 



71 



5. a) Head shield globe shaped, tail with 3 free segments 



on each side Sphaerexochus 



b) Head shield not globe shaped, tail segments 



fused together. 6 



6. a) Center of head shield squarish, tail shield with 



one short terminal spine .Trimerus 



b) Center of head shield triangular, 4-lobed; tail 



shield without spines Calymene 



Bumastus is the only common Silurian ge- 

 nus with unsegmented tail shield and smooth head 

 shield. B. niagarensis (fig. 135) is the commonest 

 species. 



Fig. 138 



Encrinurus and all the remaining genera 

 have segmented tail shields; in Encrinurus the tail shield 

 is pointed but not prolonged into a spine. E_. ornatus 

 (fig. 136) is common in the Niagaran of Ohio. 



Dalmanites has a tail shield with a single stout spine 

 on the end of it. D. vigilans (fig. 137) is the common 

 species of the Niagaran. 



Cheirurus niagarensis (fig. 138) has a tail 

 shield with 3 lateral spines, which are charac- 

 teristic. It is present in the Cedarville and 

 Guelph. 



Sphaerexochus romingeri (fig. 139) has a 

 tail shield with 3 free segments; its head shield 

 is globular and the glabella is widest in the front. 

 It is common in the Cedarville and Guelph. 



Trimerus delphinocephalus (Homalonotus in 

 older publications) is a large species (fig. 140); the 

 glabella is squarish, unlobed, and not globe-shaped; 

 there is one short terminal spine on the tail. It is 

 common in the Niagaran of Highland County. 



Calymene (see fig. 79) is smaller than Trimerus , 

 has no terminal spine on the tail shield. The glabella 

 is narrowest in front and 4-lobed. C. niagarensis is 

 is common in the Niagaran. 



Fig. 135 



Fig. 140 



f *b-13?- E CHINODE RM A . Cystoids and crinoids are common enough in some of 



our Silurian beds to require notice here. Entire specimens are rare and the 

 arms are generally not preserved. In this state, cystoids and crinoids are not easy to separate; 

 therefore, both groups have been keyed together here and treated separately afterwards. See 

 Busch (1943) for a more extensive discussion. 



Key to the Commoner Silurian Echinoderma of Ohio 



(see p. 4 for use of keys) 



1. a) Cup longer than wide 2 



b) Cup more or less globular, about as long as wide 5 



