ARTHROPODA. TRILOBITA. 



195 



GROUPS OF DOUBTFUL POSITION. 

 OEDEE. TEILOBITA. 



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; in a few genera, however (Homalonotus, Illcenus), 



..k 



FIG. 53. Calymene blumenbachi, from the Wenlock Limestone. A, head, 

 B, thorax, C, pygidium. a, glabella; a', axal furrow; b, one of the 

 glabella furrows; b', neck furrow; d, facial suture; e, eye;/, free 

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

 Natural size. 



this trilobation is only very feebly shown, or almost absent. 

 In a longitudinal direction, three regions can be distin- 

 guished in the body, the head (fig. 53 A), the thorax (B), 



132 



