356 



ARTHROPODA. 



crum" of each is often bevelled off, so as to form a facet 

 upon which the pleura immediately in front plays, thus 

 allowing the animal to roll up. In the state of complete 

 enrollment, the under surface of the pygidium is closely 

 applied to the corresponding surface of the head-shield, thus 

 entirely concealing the ventral aspect of the animal. Some 

 forms (such as Homalonotus, Lichas, Triarthrus, Olenus, Par- 

 adoxides, j^Eglina, &c.) are not known to have been endowed 

 with the power of rolling up. Though the trilobation of the 

 thorax is usually very well marked (figs. 208-211), at other 

 times the axis is very broad, and the axal furrows more or 

 less inconspicuous. This is the case in Illcemis (fig. 212), 

 and to a less extent in Homalonotus. 



The caudal shield or " pygidium " commonly called the 

 "tail " is composed of a greater or less number of segments 

 anchylosed or amalgamated. Commonly, the pygidium is 

 trilobed (fig. 211), like the thorax, and consists of a central 



Fig. 211. Asaphus Canadensis (Chap- 

 man). Lower Silurian. 



Fig. 212. Illcenus Barr- 

 iensis (Murchison). Lower 

 Silurian. 



elevated "axis" and of a marginal "limb." The limb is 

 separated from the axis by axal furrows, and usually ex- 

 hibits on its surface the lines which indicate the component 

 pleurae, as well as the longitudinal furrows on the faces of 

 these. The extremity of the pygidium is sometimes simply 



