ECHIXOIDEA. II. 



55 



side) is covered by a rather dense, uniform coat of slender primary spines, rising from a groiiiid thickly 

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



The actinostome is somewhat before tlie niiddle, a Httle sunken. It is round, covered bv an 

 outer circle of larger, irregular piates and several smaller ones inside the.se. The niouth opening is 

 excentric, near the posterior edge. (Fig. 8.) (Conip. also PI. VII. Fig. ig.) 



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

 denuded one figured PI. VI. Fig. 9I the labrum is .separated from the following ])late bv the junction 

 in the median line of the ambnlacral piates I. a. 2 and V. b. 2 (PI. \'II. Fig. ig), as is the case in P.cinc- 

 fiis; in all the other specimens these two piates do not join in the middle line and the labrnm is not 

 separated from the sternnm (Fig. S), but it is very narrow at the aboral end. The 4lh jjlate of the 

 ambulacral series I. a and V. b has an episternal widening, vvhich reaches within the subanal fasciole; 

 no other ambulacral piates reach the fasciole. As in P. ci/icfics the fasciole 

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

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

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

 it together with the three following pairs of piates (a. 7— g and b. S— -lo); in 

 P. cinctiis the periproct is surrounded by only three pairs of interambulacral 

 piates in all, viz. a. 6— S 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 piates 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 interambulacrnm. — The anterior ambula- 

 crum is short, as in P. cincttis ; the piates above the ambitus are distinctly 

 lower than those below the ambitu.s, and likewi.se they are distinctlv lower 

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



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

 double (PI. \TI. Fig. 20). This form thus differs from the other genera of the Unchiiiida- in having 

 the ambulacra .somewhat unequalh- developed. The same feature is seen in the figures of P. ci)ictiis, 

 though not mentioned in tlie description. 



The apical sy.stem (PI. \'II. P'ig. g) is like that of P. ciiictus, disjoint in the same manner. Two 

 genital pores, covered with long genital papillæ, are found in a plate joining the ocnlar plate of the 

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

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

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

 in these forms may perhaps not be exclusiveh- bound to the basal piates, the whole apical system 



' On p. 151 (Pan. Deep-Sea Ech.) .Agas.siz savs that no trace of genital opening.s could be seen, unless one of the 

 openings seen on the large interambulacral plate in continuation of the odd (interjambulacruni be a genital pore>. In the 

 light of the faet that both the corresponding pores in P. kiysuius bear genital papillæ and thus prove themselves to be ge- 

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

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

 supposition that the specimens of P cinctus (the smaller 2i™"i| are only young stages thus beconies erroneous (though it is 

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



Fig. S. Peristome, labrum 



and adjoining piates of Plex- 



cchinus hirsulus. f'i. 



