ECHINOIDEA. II. 



55 



side) is covered In- ;i rather dense, uuifonii coat of slender primarx' spines, risinjj from a ground thickly 

 covered by short miliary s])ines. — The test is not very fragile. The largest specimen is 20""" in length. 



The actinostonie is somewhat before the middle, a little sunken. It is round, covered bv an 

 outer circle of larger, irregular plates and several smaller ones inside these. The montli opening is 

 excentric, near the posterior edge. (Fig. 8.) (Comp. also PI. \'II. Fig. ig.) 



The structure of the test agrees upon the whole with that ot P. ciiicfns. In one specimen (the 

 denuded one figured PI. \'I. Fig. 9) the labrum is separated from the following plate bv the junction 

 in the median line of the ambulacral plates I. a. 2 and V. b. 2 (PL \'II. Fig. 19), as is the case in P. cinc- 

 hes; in all the other specimens these two plates do not join in the middle line and the labrum is not 

 separated from the sternum (Fig. S|, but it is ver}- narrow at the aboral end. The 4th ]>late of the 

 ambulacral series I. a and \'. b has an episternal widening, which reaches within the subaual fascicle; 

 no other ambulacral plates reach the fascicle. As in P. ciiictus the fasciole 

 encloses the inner part of the interambulacral plates 5. a. 2 — 5 and b. 3 — 6 

 (the plates a. 3 and b. 4 are completely within the fasciole). The following 

 plates, a. 6 and b. 7 are rather elongate and reach the periproct, encircling 

 it together with the three following pairs of plates (a. 7 — 9 and b. 8 — 10); in 

 P. ciiittns the periproct is surrounded by oidy three pairs of interambulacral 

 plates in all, viz. a. 6 — 8 and b. 7 — 9, according to the figures given of that 

 species. The periproct is much sunken in its k)wer part, the point where 

 the plates 5. a. 6 and b. 7 reach the lower edge of the periproct being the 

 deepest; the upper part of it is at a level with the prominent hood formed 

 bv the abactinal part of the odd iuterambulacrum. — The anterior ambula- 

 crum is short, as in P. ciiichis ; the plates above the ambitus are distinctly 

 lower than those below the ambitus, and likewise they are distinctly lower 

 tlian those of the paired ambulacra. (PI. \'I. Fig. 13, PI. VII. Fig. 20.) The 



pores of these plates are somewhat elongate verticalh", showing a distinct tendency towards becoming 

 double (PI. MI. Fig. 20). This form thus differs from the other genera of the Urechinidce in having 

 the ambulacra somewhat unequalh' developed. The same feature is seen in the figures of P. ciiichis, 

 though not mentioned in the description. 



The apical system (PL VII. Fig. 9) is Hke that of P. cinctns, disjoint in the same manner. Two 

 genital pores, covered with long genital papillae, are found in a plate joining the ocular plate of the 

 anterior ambulacrum (PL VII. Fig. 20); this plate also bears a .single madreporic pore. Evidently the 

 same is the case in P. ductus, as Agassiz .supposes'. The plate with the genital pores must jDrobably 

 be regarded as the confluent left and right anterior genital plates; otherwise, I think, the genital pores 

 in these forms nu^\- perhaps not be e.xclusiveh- bound to the basal plates, the whole apical system 



■ On p. 151 (Pan. Deep-.Sea Ech.) .\gassiz says that no trace of genital openings could be seen, unle.ss one of the 

 openings seen on the large interambulacral plate in continuation of tlie odd (interlambulacruui be a genital pore-. In the 

 light of the fact that both the corresponding pores in P. hiysuhis bear genital papillae and thus prove themselves to be ge- 

 nital pores it is certainly not too hardy to conclude that both the pores of this plate in P. cincius are likewise genital open- 

 ings. In the figure i. PI. 60 this plate bears a third small pore, quite as in hiysiitus — evidently the madreporic pore. The 

 supposition that the specimens of P cincius (the smaller 21'nm) are only young stages thus becomes erroneous (though it is 

 of course possible that the species may reach a more considerable size). 



Fig. 8. Peristome, labrum 



and adjoining plates of Plex- 



echitius hiysutus. " i. 



