100 THE VOYAGE OF H.M.S. CHALLENGER, 



species of the group, I am compelled, for the present at least, to distinguish this species 

 from Phormosoina liiculentum, although it is found associated with it. It resembles 

 Phormosoma jdacenta in the closer tuberculation of the actinal surface (PI. X.'' figs. 3, 6), 

 and differs from both Phormosoma jplacenta and Phormosoma luculentum in the greater 

 height of its coronal plates, and the presence of large primary tubercles extending both 

 in the ambulacral and interambulacral areas far towards the abactinal system (PI. X.'' 

 fig. 4). These primary tubercles carry long curved spines (PI. X."^ fig. 1), of a whitish- 

 yellow colour, and not filled with dark pigment as in Phormosoma luculentum. This, 

 however, I do not consider a character of importance, depending as it does entirely 

 upon the general colour of the test, which, in this sjjecies, is of a dirty orange-yellow, 

 with a slight violet tint. The primary spines are comparatively stouter than in 

 Phormosoma lucxdentum. 



The most important feature which distinguishes this species at once from Phormosoma 

 luculentum is the comparatively large abactinal system (PL X.'' figs. 1, 4, 8), the genital 

 openings extending well into the median interambulacral space between the upper 

 coronal plates, and the larger number of the anal plates of the anal system than in 

 Phormosoma hiculentum. On the actinal surface (PI. X.^ fig. 2) the primary spines are 

 not tipped with a solid hoof, but all end in a fleshy bag (PL X.*" fig. 10), which gives 

 these spines much the appearance of those of the actinal surface of Microi^yga. There 

 are from two to three primary tubercles on each interambulacral plate near the ambitus, 

 and about half-way towards the abactinal system. In the ambulacra! area the large 

 primary tubercles extend nearly to the abactinal extremity of the ambulacrum. The 

 lines of miliaries, extending round the test immediately at the ambitus, are more 

 prominent than in Phormosoma luculentum (PL X.*" fig. 7) ; still they do not cor- 

 respond in the interior of the test to the remarkable band noticed by Thomson in that 

 species. 



This species is also characterised by its extremely narrow poriferous zone which, even 

 on the abactinal surface, forms a more or less irregular vertical zone, composed of short 

 arcs of three pairs of pores, and placed in close proximity to the outer edge of the pori- 

 ferous zone (PL X.'' figs. 3, 4, 5, 7). The number of primary tubercles near the ambitus 

 is larger in Phormosoma bursarium than in Phormosoma luculentum. In a specimen 

 of the former measuring 28 mm., there are four primary interambulacral tubercles to each 

 plate, while in a large Phormosoma luculentum measuring 160 mm. there are only three 

 large primaries. The miliaries of the abactinal surface are somewhat larger also than in 

 Phormosoma luculentum, and less numerous ; the interambulacral coronal plates of the 

 latter species being pitted all over with miliary tubercles, sunk in the cutis of the test. 

 The ridges separating the deeply sunken areolas of the primary tubercles of the actinal 

 surface (PL X.'' fig. 6) are quite narrow in Phormosoma hursarium, while in Phormosoma 

 luculentum the primary tubercles near the ambitus are separated by flat spaces of the 



