520 



CRUSTACEA. 



with grooved pleurae, more especially, the pleurae imbricate and 

 overlap ; and the " fulcrum " 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 enrol- 

 ment, 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, Paradoxides, sEglina, &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. 370-373), 

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

 less inconspicuous. This is the case in Illcenus (fig. 374), 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 



Fig. 373. — Asaplius Canadensis (Chap- 

 man). Ordovician. 



Fig. 374. — Illcenus Barriensis 

 (Murchison). Silurian. 



or amalgamated with one another. Commonly, the pygidium is 

 trilobed (fig. 373), like the thorax, and consists of a central elevated 

 " axis " and of a marginal " limb." The limb is separated from 

 the axis by axal furrows, and usually exhibits on its surface the 

 lines which indicate the component pleurae, as well as the longi- 

 tudinal furrows on the faces of these. The extremity of the pygi- 

 dium is sometimes simply rounded, with an " entire " margin ; but 

 it may be prolonged into a shorter or longer spine or " mucro " (fig. 

 369), and the ends of the pleurae may also be extended into spine- 

 like projections (fig. 375). The number of rings in the tail varies 

 from two (Sao hirsuta) to twenty-eight (species of Amp/iion). 



