C2 ECHINOIDEA. II. 



reaching scarceh- beyond the middle of tlie first plate of the adjoining ambulacra. The sternum is 

 likewise very small, these two piates together representing what has been interpreted by Agassiz in 

 P. Rathbttni as the labrnni alone. It also appears from the remark (Paiiamic Deep-Sea Ech. p. 163) It 

 is possible that the labiiim is made up of two piates and theii foUowed b\- the regular succession of 

 piates \ that Professor Agassiz has not been quite certain of the structure in P. Rat/ibu?ii\ and I 

 think \ve may then safely conclude that P. Rathbuni agrees with P. vcsica regarding the structure 

 of the odd interambulacrum, since tliey otherwise agree in all more important features. — The plastron 

 of Pilcmatccliinits is thus in general accordance with that of Urcchinus. and the genus has a typical 

 meridosternon , differing from that of Urechimts only in the small size of the piates, as upon the 

 whole all the piates near the actinostome are mucli smaller than in Urcc/iiniis. It is worth noticing 

 that in the paired interambulacra the inner piates are quite .similar to those of the odd interambula- 

 crum, as is especially well seen in PI. 85. Fig. 2 of the Panamic Deep-Sea Ech. , when the transverse 

 line which is wanting between the labrum and sternum is added. — It may be stated expressly that 

 the pores are simjile. 



Another feature of no small interest I noticed on examining P. vesica, viz. that it has quite 

 distinct auricles; they do not form a ringshaped thickening of the piates all round the peristome, but 

 are present in the shape of five distinct elevations across the interambulacra close to the peristome, 

 ending with a somewhat more elevated portion in the middle of the adjoining ambulacral piates. They 

 are so distinct that oue might indeed be tempted to suggest the existence of a rudimentary dental 

 apparatus in this species; there is, however, no trace of it, at least in the grown speciniens, but it 

 seems not unreasonable to suggest that the embryos will show some traces thereof. 



It may further be remarked that in P. vesica the alleged resorption of the tubercles is very 

 conspicuous — but it is beyond doubt that they have not been resorbed, but only rubbed off; it is 

 very easy to rub the tubercles off, and exactly the same appearance as the resorbed tubercles is 

 produced. (Comp. above p. 41.) 



Concerning the inner anatomy of P. vesica it may be noticed that there is at least one very 

 well developed sipho. 



The pedicellariæ of this species have received some attention in the Challenger Report, three 

 different kinds being mentioned. I have found four kinds, viz. globiferous, tridentate, ophicephalous 

 and triphyllous. The globiferous pedicellariæ (PL X. Fig. 7) are like those of P. Ratlibuni, the val ves 

 ending in a single hook. The tridentate pedicellariæ are rather richly developed; Agassiz gives no 

 less than four figures of them (PL XXXV. 16, XLIII. 9 — 11), besides a figure of a single valve (PL XLV. 

 36). I have found two distinct forms of tridentate pedicellariæ; the one has the \-alves rather abruptly 

 narrowed and tlie edges inrolled in the lower part (PL X. Fig. 13); there may be some meshwork in 

 the blade. In the smaller forms the narrowed part is shorter, and quite small ones are, as nsually, 

 simply leafshaped. The largest ones seen were i""" long (head). The figures PL XXXV. 16 and XLIII. 

 9 of the »Challenger i-Echinoidea represent this form, and since the PL XLV. Fig. 36 is said in the 

 explanation of piates to be a valve of the form represented in PL XLIII. 9, this figure also belongs 



' Also in one specimen of P. vesica the transverse line between the labrum and sternum was not quite distinct; in 

 other speciniens it was beyond doubt. 



