102 THE VOYAGE OF H.M.S. CHALLEXGER. 



the abactinal system the primaries decrease in size and number, leaving the abactinal 

 portion of the test covered by secondaries and miliaries for one-third the distance from 

 the abactinal system to the ambitus. The arrangement of the tubercles is similar in the 

 ambulacral system, the tubercles being, however, somewhat smaller. The primary spines 

 are cylindrical, hollow, of uniform length (PI. XL fig. 1) ; those of the actinal system are 

 capped with a gigantic, white, truncated conical hoof (PL XL fig. 2). The spines of 

 the secondary tubercles and of the miliaries are sharp, shorter, more slender, pointed, 

 and tapering. The actinal system (PI. XL fig. 2) is thickly covered with short 

 stouter spines, carried by the secondaries, and granules covering the plates of the 

 actinal membrane (PI. XIL fig. 4). The abactinal system is less well covered with 

 spines (PL XL fig. 3), the granulation of the anal system is quite uniform in size 

 (PI. XL fig. 4), and the anal system is towards its outer edge covered by large 

 irregularly-shaped plates, with slightly raised edges separating the plates of the genital 

 ring ; these plates carry no tubercles, only an indistinct granulation. The genital and 

 ocular plates are fringed by an irregular row of miliaries (PL XL fig. 4). 



In Phormosoma, Yiewedi from the outside, and counting from the actinostome, the 

 whole up]Der edge of the plates of the ambulacra pass under the lower edge of the 

 succeeding plates, while in the interambulacral areas it is the lower edge which passes 

 under the upper edge of the preceding plate (PL XIL" figs. 11, 12). 



Seen from the actinal side, the poriferous zone cannot well be traced through the 

 mass of spines, and when denuded, is seen to extend very irregularly towards the 

 actinostome (PL XIL fig. 4 ; PL XII.--" fig. 10). 



As in PJiormosoina hursariwn, Phormosoma lolacenta, and other allied species, the 

 depressed areolas of the large primary tubercles of the actinal surface form distinct 

 projecting rings on the interior of the test (PL XIL* fig. 11). 



In alcohol the colour of the specimens of this species is dark violet, almost black both 

 for the test and spines, and this formed a marked contrast to the white tips of the spines 

 on the actinal surface. 



The existence of primary spines tipped with hoofs as in the Ai'baciadse is an 

 interesting structural feature connecting groups which thus far seemed somewhat isolated 

 in their relationship to other Echinids. Thomson in the Voyage of the Challenger (vol. i.) 

 figured these remarkable spines of the actinal surface of Phormosoma hoplacantha on 

 p. 148, fig. 35, where he speaks of the wear of the base of the cones as if they had been in 

 use for " vigorous locomotion " over the ground, as we know to be the case in one of 

 the species of Arbacia of the eastern North American coast. In the Echinothuridaa the 

 conical tip does not extend along the sides of the extremity of the spine, forming, as in 

 the Arbaciadse, a kind of cap ; it is merely attached by a nearly horizontal base to tlie 

 more flattened end of the spine. In consequence of this mode of attachment the tip is 

 frequently lost. 



