296 AN INTRODUCTION TO THE STUDY OF FOSSILS 



Isotelus (Fig. 128). Ordovician. 



Dorsal shield oval, its cephalon and pygidium large, subequal, 



with obsolete segmentation and wide flattened borders. The 



thorax has eight seg- 

 ments, with a broad 

 and rather indistinct 

 axial lobe. The name 

 was given by Dekay, 

 1824, in reference to 

 the subequal cephalon 

 and pygidium, from 

 the Greek isos, equal, 

 -H telos, end. 



Abundant in North 

 America. I. gigas, a 

 species abundant in 

 the mid-Ordovician of 

 North America, oc- 

 curs very frequently 

 with the dorsal shield 

 entire. 



1. Sketch specimen, 

 noting glabella, eye 

 lobes, thorax, pygid- 

 ium. 



2. How does this 

 differ from Paradox- 

 ides in (a) glabella, 

 {b) eye lobes, (c) axial 

 lobe of thorax, {d) py- 

 gidium ? 



3. What is the sig- 

 nificance of the name ? 



sflfS^^^^-E-f^fT^' 



B 



Fig. 128. — Isotelus gigas Dekay. Natural size. 

 A, dorsal view of a broad form, possibly a 

 female, of this species. B, a side view, e., eye ; 

 e.l., eye lobe; fa.s., facial suture ; g/., glabella ; 

 pg., pygidium. (A, after Logan; B, after Hall.) 



Trinucleus (Fig. 129). Ordovician. 



The dorsal shield, the part of the animal usually found fossil, 



possesses the following characters: the cephalon is very broad 



