CRUSTACEA. 



169 



chief genera are Olenus, Paradoxides, Dikelocephalus, Cono- 

 coryphe, Sao, Angelina, and Ellipsocephalus. 



3. AsAPHiDj. Large Trilobites, generally oval, and never 

 furnished with spines or tubercles on their surface. The eyes 

 smooth, and the facial sutures terminating on the posterior 

 margin. The cephalic and caudal shields generally of large 

 size, the glabella of the former often obscure, and the latter 

 sometimes exhibiting no indication of its component segments. 

 The body-rings usually eight in number, sometimes more, rarely 

 fewer (six in ^Sglina). 



The family Asaphidce is characteristically Lower Silurian 

 in its distribution, commencing by a few forms in the Upper 

 Cambrian, and being hardly at all represented in Upper Silu- 

 rian strata. The most important genera are Asaphus, Ogygia, 

 Illcenus, ^Eglina, Barrandia, and Psiiocephalus. 



4. TRINUCLEID^E. Cephalic shield large, the posterior angles 

 of the cheeks prolonged into long spines. Body-rings- six 

 (sometimes five ?) in number. Facial suture sometimes absent 



Fig. 115. Triune leus Pougerardi, Lower- Silurian. 



(Trimicleus) ', eyes sometimes wanting (Ampyx). The Trinu- 

 deidtz are exclusively Lower Silurian, though there are traces 

 of their existence in the higher portion of the Upper Cambrian. 

 The only genera referred to this family are Trinudeus, Ampyx, 

 and Dionide. 



5. CHEIRURID^E. Cephalic shield with the facial sutures 

 terminating on its exterior margins. Body-rings eleven. 

 Pleurae with free extremities. Caudal shield of few segments, 

 the ends of these being free. The family extends from the 

 Upper Cambrian to the Devonian, but it attains its greatest 



