530 



CRUSTACEA. 



there are only thirteen or fourteen segments in the thorax, and the 

 lateral lobes of the pygidium are undeveloped, the elongated and 

 pointed axis alone remaining. The genus Olenellus appears to be 

 characteristic of the Lower Cambrian deposits, while Paradoxides 

 distinguishes the Middle Cambrian fauna, and Olenus (or Dikello- 

 cephalus) the Upper Cambrian. 



In Hydrocephalus, again, the glabella is immensely inflated, with 

 a median longitudinal groove ; and the facial sutures cut the outer 



Fig. ^.—Paradoxides Micmac. St John's Group (Cambrian), Newfoundland. 

 (After Dawson.) 



margins of the head-shield, so that the free cheeks become much 

 reduced in size, and the genal spines are attached to the fixed 

 cheeks. Eyes are wanting, and there are only twelve body-rings. 

 A more important and widely distributed genus of this family is 

 Olenus (fig. 387), in which the general characters are very similar 

 to those of Paradoxides, but the glabella is contracted in front so 

 as to become conical, and its furrows are often extended completely 



