ASTHENOSOMA CORIACEUM. nr, 



primary plates carry along the central line a liorizontal row of tliree to five 

 prominent secondary tubercles with an occasional intercalated miliary. 

 Towards both the abactinal and actinal extremities the secondary tubercles 

 gradually decrease in number, as well as in prominence, leaving the median 

 part of the plates of the ambulacra! zone bare of tubercles and miliaries. 



Asthenosoma coriaceum A. Ag. 



Asthenosoma coriaceum A. Ag. 1879, Pioc. Am. Acad. Vol. XIV, p. 201. 

 Asthenosoma coriaceum A. Ag. 1881, " Challenger " Echinoidea, p. 88. 



Plate 52. 



As there is no very detailed figure of the abactinal system of an Astheno- 

 soma, I have given on PI. 52, fig. 1, a figure of the abactinal system and 

 adjoining coronal plates of A. coriaceum from a "Challenger" specimen 

 (fragment from 310 to 315 fathoms, " Challenger" Station 173, off the Fiji 

 islands) of 125 mm. in diameter. The abactinal specimen measured 

 38 mm. 



In none of the Echinothurice I have examined are the plates of the abacti- 

 nal system so greatly subdivided as in A. coriaceum. The ocular plates alone 

 are readily isolated ; they are crescent-shaped with truncated horns, the 

 ambulacral system entering deeply into the crescent of the plate. The 

 ocular plates vary considerably in size ; the odd anterior ocular, the largest, 

 is nearly twice as large as the smallest ocular, — the right posterior. The 

 oculars and genitals are separated by anal plates with the exception 

 of the madreporic genital, which is connected with the adjoining oculars, 

 and the right posterior one, which has a point of contact with the odd 

 posterior genital. The genitals are triangular, longer than broad, extemling 

 in the median line to the third interambulacral plate. The position of the 

 genital opening also varies considerably. In the odd posterior genital it is 

 in a bare space near the anal base of the plate; in the madreporic genital, 

 at the very tip of the genital plate and in the others, in a bare space in 

 the distal part of the plate (PI. 52, fig. 1). All the genital plates are 

 greatly broken up into distinct plates — so much so that it is impracticable 

 to distinguish the original anal plates from those which may have been 

 split off from the anal edge of the genitals (PI. 52, fig. ^). The genital 

 openings all open in a small bare part of the genital plate ; here and 



