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CRUSTACEA. TRILOBITA 



SUB-CLASS I. TRILOBITA 



The Trilobites derive their name from the fact that the 

 body is divided into three parts, by means of two furrows, 

 which extend from the anterior to the posterior extremi- 

 ties ; this trilobation is usually conspicuous, but in a few 

 genera {e.g. Homalonotus, Illamus), it is indistinct or 

 almost obsolete. The body is oval in outline, and flattened 

 from above downwards ; it consists of the head (fig. 123 A), 

 the thorax (B), and the pygidium or abdomen (C). The 



Fig. 123. Calymene tuberculata, from the Wenlock Limestone. Dorsal 

 surface. A, head; B, thorax; C, pygidium or abdomen, a, glabella ; 

 a', axial furrow ; b, one of the glabella furrows ; b', neck-furrow, 

 behind which is the neck-ring ; d, facial suture ; e, eye ; /, free 

 cheek ; g, fixed cheek ; h, genal angle ; i, axis of thorax ; k, pleura. 

 Natural size. 



