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



generally the case in that famil)^, the actinal keel forming a prominent rounded 

 keel extending from the actinostome to the extremity of the anal snout (PI. XXIII. 

 figs. 1, 4, 5, 7). 



*Ciono'brissus revinctus (Pis. XXIIL, XXXV." fig. 18 ; PL XXXIX fig. 22 ; PI. XLI. 

 figs. 41-43). 



Cimwhritisiis revinctua, A. Agassiz, 1879, Proc. Am. Acad., vol. xiv. p. 206. 



The test of tliis species is ovoid (PL XXIIL figs. 1-5), with a moderately sunken 

 anterior ambulacrum (PL XXIIL figs. 3, 5,6, 9); this is flush with the test near the 

 apical system, deepest at the ambitus (PL XXXV. fig. 18), and extends to the actinostome. 

 The actinostome is anterior to the centre (PL XXXIII. fig. 3 ; the actinal plastron is 

 strongly arched, very prominently defined near the posterior extremity of the actinal 

 surface, where it forms a rounded keel (PL XXIIL figs. 1, 7) immediately below the anal 

 snout • this plastron is closely jjacked with tuljercles, diminishing in compactness towards 

 the prominent posterior lip of the actinostome (PL XXIIL figs. 1, 9). 



Immediately round the actinostome on each side of the actinal plastron the ambu- 

 lacral areas are broad and bare (PL XXIIL fig. 9) ; the tubercles of the actinal surface 

 are largest next to the ambulacral area, and in the interambulacral zones ; they diminish 

 very gradually in size towards the ambitus (PL XXIIL fig. 1) and thence again to the 

 peripetalous fascicle. The tuljercles are quite uniformly placed on the whole test 

 (PL XXIIL figs. 1, 4, 5) with the exception of the space within the peripetalous fasciole, 

 where the primary tubercles of the interambulacral area greatly increase in size ( PL XXIIL 

 fig. 6), carrying comparatively long curved spines which completely hide the petals 

 (PL XXIIL fig. 2) ; the spines on the actinal surface are similar to those of the primary 

 tubercles within the fasciole but smaller (PL XXIIL fig. 3), while the rest of the test is 

 thickly covered by shorter curved spines (PL XXIIL figs. 2, 3). In alcohol the test is of 

 a dirty olive colour, with lighter coloured spines. 



The peripetalous fasciole is narrow (PL XXIIL fig 6), and with the exception of the 

 re-entering angle it makes before crossing the odd anterior ambulacrum (PL XXIIL fig. 5 ; 

 PL XXX v.*" fig. 18) runs almost an elHptical course round the tips of the petals. The 

 anterior pair of petals are somewhat shorter than the posterior pair (PL XXIIL fig. 6), 

 the ambulacral plates being more crowded together ; the £xterior pore in the paired petals 

 is far larger than the interior one (PL XXIIL fig. 6). 



The subanal fasciole is broad, vertically elliptical, pointed towards the actinal plastron 

 (PL XXIIL figs. 1, 4, 7). The anal system is small, circular (PL XXIIL fig. 4), placed 

 well up near the aljactinal surface. The structural apex is anterior. There are four 

 genital pores (PL XXIIL fig. G) enclosing a madreporic body, which extends into the odd 

 posterior interambulacral space. 



Seen in profile the test is abruptly rounded from the actinostome ; at the anterior 



