ECHINOIDEA. 11. 



55 



sulci is covered by a rather dense, uniform coat of slender primary spines, rising from a ground thickly 

 covered by short miliary spines. - The test is not very fragile. The largest specimen is 20™" in length. 



The actinostome is somewhat before the middle, a little sunken. It is round, covered by an 

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

 excentric, near the posterior edge. (Fig. 8.) (Com}), also PL VII Fig. 19.) 



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

 denuded one figured PL VI Fig. 91 the labrum is separated from the following plate by the junction 

 in the median line of the ambulacra] plates I. a. 2 and V. b. 2 (PI. VII. Fig. 19), as is the case in I', duc- 

 tus: 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. 8), but it is very narrow at the aborai end. The 4th plate of the 

 ambulacral series I. a and Y. b has an episterual widening, which reaches within the subanal fasciole; 

 no other ambulacral plates reach the fasciole. As in P. ductus 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. ductus the periproct is surrounded by only 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 lower 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 

 by the abactinal part of the odd interambulacrum. — The anterior ambula- 

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

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

 than those of the paired ambulacra. (PI. VI. Fig. 13, PI. VII. Fig. 20.) The 



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

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

 the ambulacra somewhat unecpially developed. The same feature is seen in the figures of P. cinctus, 

 though not mentioned in the description. 



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

 genital pores, covered with long genital papilke, 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 1 . The plate with the genital pores must probably 

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

 in these forms may perhaps not be exclusively bound to the basal plates, the whole apical system 



1 On p. 151 (Pan. Deep-Sea Bch.) Agassiz says that 110 trace of genital openings could be seen, unless one of the 

 openings seen on the large interambulacral plate in continuation of the odd (inter)ambulacruru be a genital pore . In the 

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

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

 ings. In the figure 1. PI. 60 this plate bears a third small pore, quite as in hirsutus — evidentlv the madreporic pore. The 

 supposition that the specimens of P ductus the smaller 2i mm l 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- 



echinus hirsutus. 9 ,. 



