352 



ARTHROPODA. 



common for the same species of Trilobite to present itself 

 under two forms, one broad and the other long, and he 

 regards the broad forms as the females, and the long forms 

 as the males, of the species. 



The crust exhibits a well -marked division into three 

 regions, which are commonly found detached and separate 

 from one another. These three regions are 1, a cephalic 

 shield ; 2, a variable number of movable " body-rings " or 

 thoracic segments ; and 3, a caudal shield or " pygidium." 

 The cephalic shield or buckler (fig. 206) is generally more 



Fig. 206. The skeleton of a Trilobite (Angelina Sedgwicldi), parliully dissected A, Head- 

 shield. B, Movable rings of the thorax, c, Tail or abdomen, g, Glabella, in this species 

 without furrows ; fi, Fixed cheeks ; e, Eye-lobe ; o, Eye ; /, Facial suture ; fr, Free cheeks ; 

 s, Head-spines ; p, Pleura ; pp, Anchylosed pleurae of pygidium. 



or less semicircular in shape, and is composed of a central 

 and two lateral pieces, of which the two latter may or may 

 not be united in front of the former. The central portion 

 of the cephalic shield is usually elevated above the remainder. 

 It is termed the "glabella" (fig. 206, g), and it protected the 



