40 S. LOVEN, ON POURTALESIA, A GENUS OF ECHINOIDEA. 



the sphei'ids, mostly uncovered, — Lovenia alone having theiu recondite, — ai'e often 

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

 ])hyllodean pedicels. Thus they generally are fewer on the front ambulacrum, III, in 

 :i greater number, though arranged in the same manner, on the two ]3aired ambulacra 

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

 disposed, on the liivium, 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 law 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 h, 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 Echinidaj^), and grown over in the Cassidulida'"). Consequently, in the Sjiatan- 

 gidaa, 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 different in Pourtalesia Jeffreysi. There are only foui' spherids, 

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

 of the bivium, I 1 and V 7, 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 tAvo other are placed on the plates W a 1 and IV b 1 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 Spatangida^ they agree 

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

 in the first spherids of the j'oung 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, / and V, /, the former represents the two peristomals I a 1, \ b 1, and the latter 

 the two V a 1 and Y b 1, in other Echinoids, and as these two jjairs are ahvays sym- 

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

 as Avith 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 

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

 symmetrically towards eacli other, as detailed above, ji.g. 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 part 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 84 

 mm. one of the larger spherids measures 0,i'C mm. in length, and 0,-'i! mm. in trans- 

 verse diameter, and one of the smaller 0,2;{ mm. and 0,vu mm., in the same dimensions. 

 Compared to a spherid, 0,w nun. in length, taken fi-om a specimen of Meoma ventri- 



') Etudes, p. .37, pi. XVTI, fig. 141—147. -) lb. p. ;j(;. pi. VII. W^. Gl— 66. ■') lb. pi. Ill, fig. 32, 

 3.3, 34. 



