278 BULLETIN 100, UNITED STATES NATIONAL MUSEUM 



station 204 they are all insert. The genital pores are rather distant 

 from the edge. The periproctal plates are few, and the whole peri- 

 proct is small. All the apical plates are closely covered by tubercles 

 of uniform size (fig. 15). 



The peristome is markedly smaller than the apical system, only 

 from 40 to 41.7 per cent of the horizontal diameter as against from 

 50 to 55 per cent of the horizontal diameter, and quite flat. There 

 are eight or nine ambulacral plates in a series. The ambulacra do 

 not quite join at the mouth edge; there are three or four interradial 

 plates in a series. 



The ambital spines are about one and one-half times the horizontal 

 diameter (it appears that the point is broken in most of those of the 

 Albatross specimen) . They are slightly fusiform at the base, more so 



Fig. 15.— Part of apical system of Schizocidaris serrata (Mortensen). X8 



in the Challenger specimens, then taper very gently to a rather fine 

 point. They are beset more or less closely with anteriorly directed 

 but usually almost straight spinules, two lateral series of which may 

 be somewhat more conspicuous than the rest; but generally an 

 arrangement of the spinules in longitudinal series is very indistinct. 

 In general the spinules are in the main confined to the adapical side 

 of the spine; more rarely they are also well developed on the adoral 

 side. The point of the spines is not widened into a small crown. 

 The surface of the spines is apparently quite smooth and shining; in 

 reality it is covered by a coat of hairs, which anastomose at their 

 ends so as to form a complete, closed, very finely fenestrated roof, 

 which rests upon the basal part of the hairs as upon columns (figs. 

 16, a-c) . The apical spines are not different from the ambital, merely 

 somewhat shorter. The oral primaries are slender, smooth, merely 



