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, 

 + 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, 

 {h) eye lobes, {c) axial 

 lobe of thorax, {d) py- 

 gidium ? 



3. What is the sig- 

 nificance of the name ? 



-ro^s^^'E-irTTT 'f' 



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

 A, dorsal view of a broad form, possibly a 

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

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

 pg., pygidium. (/I, 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 



