148 



PANAMIC DEEP SEA ECHINI. 



Plexkchinus A. Ag. 



Plexechinua A. A.g., Bull. M. ('. Z. 1898, XXXII. No. 5, p. 76. 



This genus combines features of the Ananchytid genera Urechinus and 

 Palaeotropus, while possessing also striking features of the Pourtalesiae. It 



lias, like Urechinus, a long hexagonal plastron, a 

 flush actinostome, with the primordial and acti- 

 nal ambulacra! plates in the same position as in 

 that genus, Fig. 214, rudimentary phyllodes, and 

 a well developed subanal fasciole surrounding the 

 proboscis; in Urechinus the fasciole is rudimen- 

 tary, the general arrangement of the tubercles 

 both on the actinal and abactinal sides is similar 

 to that of Palaeotropus. We may look upon the slightly sunken anal sys- 

 tem and the protrusion of the anal rostrum surrounded by its fasciole as 



\ 



'jo nun. 



Fig. 214. Plexechintjs cinctus. 



Fig. 215. Schizaster fragilis. 



After Loten. 



Fill. 216. El HINOI LRDIUM COBDATUM. 



After Lovew. 



an early stage in the formation of the prominent anal snout so greatly 

 developed in Pourtalesia. 



The formation of an anal snout in Spatangoids, varying from such rudi- 

 mentary ones as in Brissus and the like, to such exaggerated types as the 

 snout of Pourtalesia, is connected with the constriction of the subanal 



