PLEXECHINUS. 



149 



plastron taking place between the episternal plates and plates 4 of the odd 

 interambulacrum. Where no subanal plastron, or only an anal faseiole 

 exists, this constriction does not take place and the plates of the odd inter- 

 ambulacrinn extend laterally so that the outline of the anal part of the 

 test becomes more or less cylindrical. An excellent example of this can 

 be seen on comparing the odd interambulacrum of Schisaster fragilis, 

 Fig. 215, with that of Echinocardium cordafum, Fig. 216, the former having 

 an anal faseiole, the latter a subanal plastron. The effect of this constric- 

 tion to form the extreme development of the snout, as in P. Jejf'reyd and 

 P- Tunneri, is shown in Figs. 171, 174. The reduction of the sternum 



a M i 



a *• t 

 Outside view. 20 mm. 



Fig. 217. Pi.exechinus oinctus. 



Inside view. '20 mm. ? (fragment), 

 FlCi. 218. P. CINCTl s. 



and of the plates of the odd interambulacrum to a diminutive size, as in 

 Uchinocrepis setigera and Spaiagocyslis Challengeri, Figs. 167, 175, reduces the 

 snout to a mere point. 



The abactinal system, Figs. 217, 218, is much like that of P. Tanned 

 and P. laguncula, both the zones of the posterior interambulacra separating 

 completely the trivium from the bivium, with a number of intercalated 

 plates, Fig. 217, which may belong to the odd interambulacrum, while in 

 Urechinus, Fig. 219, and Calymne the bivium and trivium are only sepa- 

 rated by the anterior zone of the posterior interambulacra, and in Palreo- 

 tropus, Figs. 245, 246, 247, the abactinal plates of all the interambulacra 

 abut against the single central genital plate. 



