ECHINOIDEA. II. 



reaching scarcely 'beyond the middle of the first plate of the adjoining ambulacra. The sternum is 

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

 P. Rathbuni as the labrum alone. It also appears from the remark (Panamic Deep-Sea Ech. p. 163) <It 

 is possible that the labium is made up of two plates and then followed by the regular succession of 

 plates, that Professor Agassiz has not- been quite certain of the structure in P. Rathbuni 1 , and I 

 think we may then safely conclude that P. Rathbuni agrees with P. vesica regarding the structure 

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

 of Pilcmatechinus is thus in general accordance with that of Urechinus, and the genus has a typical 

 meridosternon , differing from that of Urechinus only in the small size of the plates, as upon the 

 whole all the plates near the actinostome are much smaller than in Urechinus. It is worth noticing 

 that in the paired interambulacra the inner plates 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 simple. 



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 plates 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 ambulacra! plates. They 

 are so distinct that one 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 specimens, 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 pedicellarise 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 pedicellarise (PI. X. Fig. 7) are like those of P. Rathbuni, the valves 

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

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

 36). I have found two distinct forms of tridentate pedicellariae; the one has the valves rather abruptly- 

 narrowed and the edges inrolled in the lower part (PI. 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 usually, 

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

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

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



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

 other specimens it was beyond doubt 



