5i6 



CRUSTACEA. 



of seven inches across the carapace. Various Devonian fossils have 

 been placed, under special generic names, in the immediate neigh- 

 bourhood of Discinocaris, and the Triassic genus Aspidocaris is also 

 regarded as closely allied to this genus. 



Order IV. Trilobita. — The Trilobites are Palaeozoic Crusta- 

 ceans in which the body is more or less distinctly trilobed, and the 

 head, thorax, and abdomen are distinct. The head is covered with a 

 cephalic shield, which usually bears a pair of compound sessile eyes, 

 ocelli being apparently never developed. The somites of the thorax are 



Fig. 368. — The skeleton of a Trilobite {Angelina Sedgivickii), partially dissected, a, Head- 

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

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

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



movable upon one another, and carry jointed legs to which branchice 

 were attached. The abdominal segments are fused to form a caudal 

 shield, and support below jointed appendages similar to those of the 

 thorax. A well-developed upper lip (" hypostome") is present, and 

 some of the cephalic appendages are modified to act as mouth-organs. 



As regards the general structure of the Trilobita, the body was 

 protected by a well-developed calcareous shell or " crust " which 

 covered the whole dorsal surface of the body, and which usually 



