ASTHENOSOMA PELLUCIDUM. 113 



as well and as regularly developed as the arnbulacral buccal plates. The 

 structure of the actinal system is essentially Echinid. The primordial 

 interarabulacral plates are large tor the size of the test (PI. 50, fig. 1). From 

 the irregularity and great size ot' the actinal arnbulacral plates it is difficult 

 to trace the limits of these areas. The actinal plates of both systems are 

 fairly well covered by prominent secondaries placed close together (PI. 50, 

 fig. 1). Between many of the plates linear or polygonal spaces are left 

 bare; this adds greatly to the apparent irregularity of their arrangement. 

 The abactinal system measures 12 mm. in diameter (PI. 5(1, tig. >). The 

 anal system is irregularly polygonal, and is characterized by the small num- 

 ber of its comparatively large, widely separated, and disconnected plates, 

 several of which are nearly as large as the smaller genital plates, and 

 carry one to two miliaries. It would seem as if some of the anal plates 

 were derived from the breaking up of the anal edge of the genital plates. 

 The ocular plates (PI. 50, figs. :?, 3) are remarkable for their great elongation, 

 they are practically rectangular, carrying an accumulation of miliaries 

 near the anal extremity. They are widely separated from their adjoin- 

 ing genitals by two to three intercalated anal plates, and also separated 

 by bare spaces ; with the exception of the two young interambulacral 

 plates adjoining the odd anterior ocular and the right posterior, it would be 

 difficult to distinguish the more recently formed interambulacral plates 

 from the intercalated anal plates of the other areas. The madreporic 

 genital is somewhat heart-shaped, well developed, with a small genital 

 opening. The other genitals are small, irregular in shape, either triangular 

 or even rectangular. 



Asthenosoma Grube. 

 Asthenosoma pellucidum A. Ag. 



Asthenosoma pellucida A. Ag. 1879, Proa Am. Acad. Vol. XIV, p. 200. 



Asthenosoma pellucidum A. Ag., "Challenger" EcMnoidea, p. 85, 1881. 



Plate 51, figs. 5-13. 



In a small specimen of A. pellucidum from "Challenger" Station 102, 

 34 mm. in diameter, the actinal system measured 9.5 mm. and the abactinal 

 9 mm. The smaller specimens of this species are interesting as showing, with 

 the increase in size, the gradual extension from the actinal system towards 



8 



