104 PANAMIC DEEP SEA ECHINI. 



The buccal plates of Ph. panamense are in striking contrast with those 

 of Ph. hispidum ; the former are narrow elongate, Fig. 147, as compared 

 with the broad, high, ambulacral buccal [dates of the latter. 



Above the ambitus, Fig. 148. the greater number of the large ambulacral 

 plates are excluded from the outer edge ; this is formed by the larger of 

 the smaller ambulacral plates. The inner poriferous plates are small, 

 triangular, and separated by a tongue of the larger excluded primary ambu- 

 lacral plate. In a Ph. hispidum of about the same size not more than five 

 or six primary ambulacral plates above the ambitus are thus excluded from 

 the outer edge ; in Ph. panamense there are from nineteen to twenty. 



There is no species of Phormosoma which shows to such an extent the 

 splitting up of the primary plates both in the ambulacra! and interambula- 



6 1 



130 in tii 



Fk;. 1-11). Phormosoma panamense. 



oral areas. While this can be seen to a certain extent from the exterior 

 by the fine irregular lines crossing the primary plates, in Fig. 148, in all 

 directions, it is best seen from the interior, Fig. 149, where the secondary 

 plates lap more or less and their edges are raised. The splitting up extends 

 to the larger secondary ambulacral plates. 



The abactinal system of Ph. panamense, Fig. 150, is marked for the 

 elongated outline of the ocular plates, which in Ph. hispidmn are quite 

 pentagonal. The genital plates are also smaller than those of Ph. hispidum, 

 and the genital pore is placed in the upper part of a narrow membraneous 

 slit which separates the second or third interambulacral plates. The madre- 

 poric genital exceeds greatly in size the other genitals, and the madreporite 

 extends over two of the adjoining anal plates. 



There are from three to four concentric rows of small polygonal anal 

 plates carrying from one to two miliaries according to the size of the plates. 



