370 



ARTHKOPODA. 



there are eight to twenty -two body -rings, with grooved 

 pleurae ; and the eyes are smooth. 



In Proetus itself (fig. 228, 

 c) the head -shield is semicir- 

 cular; 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 Lower Silurian 

 to the Carboniferous. Cyphaspis 

 (fig. 228, B), of the Silurian, 

 differs from the preceding chief- 

 ly 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. 

 228, A), also Silurian, has its 

 glabella much shortened, while 

 the body-rings are as many as 

 twenty -two in number. Car- 

 mon is another Silurian genus 

 allied to Proetus, but it has 



neither eyes n-or facial sutures, and it possesses eleven body- 

 rings. The genus Harpides (apparently = Erinnys of Salter) is 

 an interesting type, which appears to be intermediate between 

 the Proetidce and Olenidce, and which carries back the range 

 of the former into the Upper Cambrian. It has the " limb " 

 of the head-shield very wide, and covered with a network of 

 radiating and bifurcated nervures. On the other hand, the 

 Proetidce are represented in the Carboniferous rocks, not only 

 by Proetus itself, but also by the genera Phillipsia, Griffith- 

 ides, and Br achy met opus, some of which have been detected 

 in North America in deposits of the age of the Coal-measures, 

 and which are the most modern examples of the order Trilo- 



Fig. 226. Honialonotus delphinocephalus. 

 Upper Silurian. 



