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



side, a narrow plate, 1 and 4, is seen keeping apart the IV from the V, and the II from the 

 I, fi.g. 48, and this plate oii the left side is pushed forward so as to approach the flexure 

 and to compress and reduce the IV h 1 ; on the right side it keeps farther back, and the 

 II a y is fully developed. 



Echinocrepis cnneata Al. Ag., Pl. VII, fi.g. 53, repeats the mode of disposition 

 of Pourtalesia ceratopyga, the interradial 5, 1 separating the two bivious ainbulacra, 

 the interradials 1 1 and 4 1 being placed between I and II and between V and IV, and all 

 the four aiubulacral plates on either side being uniporous. 



Pourtalesia carinata Al. Ag., PL VI, Jig. 42 — 46; VII, 47, presents the same 

 arrangement, a small and narrow interradial plate intervening on either side between 

 the first pair of 1 and that of II, as also between the a, h 1 oi Y and of IV. Here also, 

 as in Pourtalesia laguricula and Pourtalesia ceratopyga, but unlike Pourtalesia Jeffreysi, 

 the I and V each commence with two plates, and the I a and V b are biporous, as accor- 

 ding to rnle. But, what is more, it is plainly shown in the fragment examined, that 

 the 5 of V is not separated from its plate 1 by the intervenience of the interradials 4, 

 but really contiguous to it, fig. 42, 46, part of the interradial 4, 2 coming into view 

 between V a and IV 6. It cannot be doubted that the like might be seen on the right 

 side, if the specimen were entire. This diversity is of great moment. The plates I a, 



II a. III b, IV a, V b are biporous, while the I b, II b, III a, IV 6, V a are uniporous, 

 Avhich is in eomplete accordance with the general rule. If I have succeeded in foUowing 

 rightly the very indistinct continuation of the plates beyond the curvature, the labrura, 

 ö 1, ought to be extended to the peristome, having all along on either side the I <i 

 and V b, while of I b and V a the pointed terminations alone are to be seen just 

 inside the margin of the niche, and still less of II a and IV b, the II b and IV a end- 

 ing in the very bend, so that no trace of them is visible inside. 



From all this it appears that the old order, reigning amoug the ambulacral plates 

 of the peristome and their pedicellar pores, is not all at once abandoned by the Pour- 

 talesiadaä. It is exchanged for a new order only revei-sibly and, so to speak, hesitatingly. 

 Pourtalesia carinata retains unaltered the otherwise universal Echinoidean formula: 



I a, II a, III b, IV a, V b: biporous; 



I b, II b, III a, IV b, V o; uniporous, 

 while Pourtalesia Jeffreysi, Pourtalesia laguncula, Pourtalesia ceratopyga, and Echino- 

 crepis cuneata, all present a singie pore only in each of the peristomal plates I, II, IV, 

 V, which thus, in that i-espect, become symmetrical on either side of the labrum, the 



III alone remaining as of old. In none of the five species examined are all the inter- 

 radials admitted into the peristome. In four of them the labrum, of considerable 

 size, intervenes between I and V, and in three of these the interradials 1 and 4 tend 

 to separate the I from the II, and the V from the IV. In one species alone, Pourta- 

 lesia Jeffreysi, the labrum, 5 1, is greatly reduced, and the I and V close with one 

 another behind it, so as to exclude it completely from the rest of the perisomal system. 

 All this recalls in a cei'tain degree what is seen among the Clypeastridte, another type 

 of late appearance. In these ^), of the peristomal ambulacrals none are biporous, all 



1) Études p. 32, pl. XLIV— LII. 



