40 S. LOVEN, ON 1'0URTALESIA, A- GENUS OF ECHINOIDEA. 



the spherids, mostly uncovered, — Lovenia alone haviiig theui recondite, — are often 

 inore or less unequally disposed, generally with some relation to the distribution of the 

 phyllodean pedieels. Thus they generally are fewer on the front ambulacrum, III, in 

 a greater number, though arranged in the same nianner, on the two paired arnbiilacra 

 of the trivium, II and IV, and most numerous, and not seldom somewhat differently 

 disposed, on the bivium, as may be seen more particularly in Moira, Schizaster, Bris- 

 sus, Plagionotus. Appertaining, as they do, to the ambulacral system, they obey, in 

 their mode of appearance during the development of the individual, the laAv govern- 

 ing the growth of that system. Accordingly, in the very young animal the first 

 spherid appears on each of the plates I b, II b, III a, IV b, V a, the second on plates 

 I a, II a, III b, IV a, V b, and so forth, and in the same order also they are shed 

 in the Echinidse '^), and grown över in the Cassidulidaa^). Consequently, in the Spatan- 

 gida3, the plates bearing the first spherids are those having a single pedicellar pore, 

 while the second spherids appear on the biporous plates. 



All this is very dififerent in Pourtalesia Jeffreysi. There ai'e only four spherids, 

 Pl. IV, fig. 15. Of these the two are placed one on each of the two first single plates 

 of the bivium, I 1 and V 1, not far behind the pedicellar pore, close to the external 

 suture, in a depression partly extending to the adjoining plate of the trivium. In the 

 same manner the two other are placed on the plates II a 7 and IV 6 i of the tri- 

 vium. They are all sub-globular, those of the first pair somewhat larger than those 

 of the second, and all are uncovered. With the spherids of the Spatangidas they agree 

 also in their proximity to the suture, and in the leaning över it, a feature observable 

 in the first spherids of the young of these, Pl. XV, fig. 172, 174, 175, but soon lost^). 

 The plates II b 1, IV a 1, and the whole of III, are devoid of spherids. Of the plates 

 1, 1 and V, 1, the former represents the two peristomals \ a 1, \ b 1, and the latter 

 the two Val and V b 1, in other Echinoids, and as these two pairs are ahvays sym- 

 metrical towards one another on both sides of the middle line, in their joint outline 

 as with regard to their component elements, the position of the two larger spherids 

 in Pourtalesia Jeffreysi is fully in harmony with the general law. But, according to 

 that law, the first spherids of the ambulacra II and IV ought to have appeared on II 

 b and IV b, that is to say: unsymmetrically, whereas in this Pourtalesia they are placed 

 symmetrically towards each other, as detailed above, fig. 15. These spherids, there- 

 fore, like the rest of the ambulacral elements, are disposed solely in relation to the 

 actual antero-posterior axis of the skeleton. They are also developed exclusively on 

 that reduced pai't of its ventral surface which is in close contact with the ground on 

 which the animal lives. Their size is relatively considerable. In a specimen of 34 

 mm. one of the larger spherids measures 0,26 mm. in length, and 0,2u mm. in trans- 

 verse diameter, and one of the smaller 0,23 mm. and 0, 20 mm., in the same dimensions. 

 Compared to a spherid, 0,20 mm. in length, taken from a specimen of Meoma ventri- 



1) Études, p. 37, pl. XVII, fi^. 141—147. -) Ib. p. 36. pl. VH, fi^. 61—66. ^) Ib. pl. III, fig. 32, 

 33, 34. 



