536 



CRUSTACEA. 



the axal furrows are distinct ; but in others (Bumastus), the thoracic 

 axis is extremely wide, and is hardly separated from the pleurae by 



Fig- 394- — A > A complete example of Illcenus Davisii, in its unrolled state— Ordovician ; b, 

 Hypostome of the same ; c, Illcenus (Bumastus) Barriensis, rolled up— Silurian. (After 

 Salter.) 



recognisable axal furrows (fig. 374). In the former of these sections 

 there are ten body-rings ; whereas in Octillcenus and Panderia there 



Fig. 395. — A, sEglina prisca — Ordovician; B, Head-shield of Cheirurtis juvenis— Ordovician ; 

 c, Head-shield of Amphion Fischeri — Ordovician; d, Side view of the head-shield of Sphcer- 

 exochiis mirus — Ordovician and Silurian. (After Barrande and Salter.) 



are only eight body-rings, and in Dysplanus there are nine. The 

 species of Illcenus are exclusively confined to the Ordovician and 

 Silurian periods. 



