138 



PANAMIC DEEP SEA ECHmi. 



odd posterior ambulacrum, and stand surrounded by the abactinal plates of 

 the odd ambulacrum. 



The arrangement of these abactinal plates in other genera of Pourta- 

 lesia? does not seem to give us definite conclusions regarding their character, 

 and thus far no very young Pourtalesiae liave been examined with reference 

 to this point. In Echinocrepls cuneaia^ the odd interambulacrum is excluded 

 from the genital plate by the oculars of the posterior ambulacra, which 

 are in contact with it, in all the other interambulacra the two abactinal 

 plates touch the genital. The oculars of the trivium are wanting. A very 

 different condition of things exists in E. setigera, as will be seen later. 



Fio. 200. P()(RTAi,ESiA Jeffreysi. 



After Lovto. 



15 mm. 

 FlO. 201. POUBTALESIA TANNERI. 



P. Tanneri, although not more than 16 or 17 mm. in length, is evidently 

 an adult, while P. Jeffreysi is fully 45 mm. long. 



The subanal fasciole of P. Tanneri is, when seen facing the anal ex- 

 tremity, somewhat pentagonal, Fig. 201 ; it is mainly limited to the plates 

 of the odd interambulacrum and does not extend laterally so as to include in 

 the subanal plastron an}'^ large part of the plates of the adjoining ambulacra, 

 as is the case in many Spatangoids, in which narrow parts of the outer 

 edge of the ambulacral plates extend far into the plastron, as in Brissus, 

 Spatangus, Echinocardium, Breynia, Maretia, Paleopneustes, and others, 

 showing a tendency to the formation of a subanal snout. In Cionobrissus 

 the subanal snout is, as in P. Tanneriy bound by a subanal fasciole restricted 

 in great part to the odd interambulacral plates. In Plexecliinus the subanal 

 fasciole surrounding the snout is, with the exception of one ambulacral 



^ Loven, Pourlalesia, PI. VII, fig. 54. 



