98 THE VOYAGE OF H.M.S. CHALLENGER. 



but few in number between the secondary tubercles of the inner part of the test where 

 the white lines separating the coronal plates are again quite as prominent as on the 

 abactinal surface (PI. X. fig. 1), while on the outer third of the test towards the ambitus 

 the sutures of the plates are concealed by the thickened cuticle extending over the 

 scrobicular area of the large primary tubercles, and by the crowded miliaries and 

 secondaries. The miliaries, though more crowded, do not form (PI. X.'' fig. 4) as in 

 Phormosoma 2:>lacenta the very characteristic edge running along the - ambitus as 

 described by Thomson, which takes almost the prominence of a fasciole (see also 

 Phormosoma htrsarium for the description of a similar fasciole), and is interesting as 

 showing how such a structure may exist in a rudimentary form in the Desmosticha. 



Both in this species and in Phormosoma bursarium (PI. X.*) the differences between 

 the primary spines of the actinal and abactinal surface is very striking. In the one 

 {Phormosoma luculenta) we have the actinal surface with its large hollow primary spines 

 tipped with the enormous white conical hoof (PI. X. fig. 1) and the small sharp or 

 club-shaped miliaries and secondaries, while the abactinal surface carries long slender 

 curved primary spines gradually tapering to a point with short, sharp secondary and 

 miliary spines (PI. IX. fig. 1). In the other {Phormosoma bursarium) the general 

 appearance of the abactinal spines (PI. X.*") is quite similar to that of Phormosoma 

 luculentum, while on the actinal side we find the remarkable primary spines with the bag- 

 like terminal appendage simulating the hoof of the other Echinothuridae. In Phormosoma 

 luculentum a few of the secondary ambulacral and interambulacral spines near the apical 

 system resemble the peculiar sheathed spines so characteristic of the whole of the 

 abactinal surface of Asthenosoma grubii (PI. X." figs. 5, 6). 



The actinal membrane is covered by secondary tubercles arranged in irregular 

 concentric rings round the actinostome (PI. X. fig. 3 ; PI. X." fig. 4), carrying slender 

 spines slightly club-shaped at the end. The abactinal system is characterised by the 

 large size of the anal plates extending from near the central part of the anal system to 

 the edge of the abactinal system (PI. X. fig. 2 ; X.* fig. 3) ; each of these iiTegularly- 

 shaped plates carries from one to two slender secondary spines. The genital plates are 

 small and elUptieal, widely separated from the indistinct ocular plates by the plates of the 

 anal system encroaching upon the genital ring. The madreporic body is not prominent. 



In this species, as in Phormosoma hoplacantha, the continuation of the poriferous 

 zone from the abactinal side to the actinostome is exceedingly irregular (PI. X. fio-. 1 ; 

 PL X.'' fig. 4), and although on the actinal side the sutures of the ambulacral and interam- 

 bulacral plates and of the poriferous zone are well defined, the poriferous zone never 

 becomes prominent and regular again as on the abactinal side. 



Thomson has already noticed the very irregular course of the poriferous zone on the 

 actinal surface of Phormosoma (Porcupine Echinoidea, Trans. Roy. Soc, 1874). 



The outline of this species seen in profile is probably more nearly correct than that 



