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



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

 1, fiq. 48, and this plate on the left side is pushed forward so as to approach the flexure 

 and to compress and reduce tiie IV h I ; on the right side it keei)S farther back, and the 

 II rt y is fully developed. 



Echinocrepis cuneata Al. Ac;., PL VJl, fiy. 53, rei)eats the mode of disposition 

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

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

 the four arobulacral plates on either side being uniporous. 



Pourtalesia carinata Al. Ag., Fi. VI, jig. 42 — 46; VI], 47, presents the same 

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

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

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

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

 ding to rule. 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, fuj. 42, 46, part of the interradial 4, 2 coming into view 

 between V a and IV /;. It cannot l)e doubted that the like might be seen on the right 

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



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

 which is in complete accordance with the general rule. If I have succeeded in following 

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

 .") 1, ought to be extended to tiie peristome, having all along un either side the 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 11 a and IV b, the II b and IV n 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 among the ambulacral [)lates 

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

 ralesiadas. It is exchanged for a new order only reversibly and, so to speak, hesitatingly. 

 Pourtalesia carinata retains unaltered the otherwise universal Echinoidean tornmla: 



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

 ] b, 11 b, III a, IV b, V a: uniporous, 



while Pourtalesia -left'reysi, i'ourtalesia laguncula, Pourtalesia ceratopyga, and Kchino- 

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

 V, which thus, in that respect, become symmetrical on either side of the labrum, the 



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

 radials admitted into tlie peristc^me. In four of tliem 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 I, 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 certain degree what is seen among the Clypeastridaj, another type 

 of late appearance. In these'), of tlie peristonuil ambulacrals none are biporous, all 



') Etudes p. 32, pi. XLIV— LII. 



