542 



CRUSTACEA. 



The species included under this head are Silurian and Devonian. Dal- 

 manites (or Dalmania) includes forms with the pleurae pointed and bent 

 backwards, the pygidium being conspicuously segmented, and often end- 

 ing in a spine (fig. 403). There are three glabella-furrows on each side, 

 and the genal angles of the head-shield are long and produced. The 

 species of this group are principally Silurian, but some of the forms are 

 Ordovician and others are Devonian. Chasmops includes Ordovician 

 species in which the genal angles are produced ; the anterior glabella- 

 lobes are laterally extended ; the pleurae have truncated ends ; and the 

 pygidium is of large size. Pterygometopus also includes Ordovician 

 species, in which the frontal lobes of the glabella are laterally extended, 

 and are intersected by the facial sutures. In Acaste are comprised 

 Ordovician and Silurian species in which the glabella is not tumid, and 

 exhibits all its furrows in a well-developed form, while the pygidium is 

 often pointed. Lastly, Cryphaus includes Devonian types, and is princi- 

 pally distinguished by the fact that the margin of the pygidium is toothed 

 or serrated. 



Family 17. Proetid^. — In this family the body is oval, dis- 

 tinctly trilobed, and capable of being rolled up. The head is of 

 variable size, semicircular, sometimes with 

 rounded, sometimes with spinose genal 

 angles. The glabella is distinctly marked 

 off from the cheeks, and the hindmost pair 

 of glabella-furrows commonly circumscribe 

 a basal lobe. The facial sutures cut the 

 anterior margin of the head-shield separ- 

 ately. There are from eight to twenty- 

 two body-rings, with grooved pleurae, and 

 the pygidium is distinctly segmented, and 

 usually has an entire margin. The eyes 

 have distinct facets, but are covered by a 

 smooth cornea. 



In Proetus itself (fig. 404, c) the head- 

 shield is semicircular ; the glabella has 

 three pairs of lateral furrows ; the eyes 

 are large, semicircular, of numerous facets, 

 covered by a thin cornea ; there are eight 

 or ten body-rings, and the tail has an 

 " entire " border. The genus ranges from 

 the Ordovician to the Carboniferous. 

 Cyphaspis (fig. 404, b), of the Ordovi- 

 cian, Silurian, and Devonian rocks, differs 

 from the preceding chiefly in its more 

 convex glabella, with circumscribed basal lobes, its ovoid and 

 remote eyes, and the generally greater number (fifteen to seventeen) 

 of the body-rings. Arethusina (fig. 404, a), with a similar range 

 in time, has its glabella much shortened, while the body-rings are as 



Fig. 404. — A, Head - shield of 

 Arethusina Konincki; b, Head- 

 shield of Cyphaspis Burvieisteri ; 

 c, Head-shield of Proetus Bohemi- 

 cus. From the Silurian rocks of 

 Bohemia. (After Barrande.) 



