34 S. LOVEN, ON POURTALESIA, A GENUS 0¥ ECHINOIDEA. 



The tliird, no less essential characteristic, the disposition of the ten ambulacral 

 rows, not in conformity with the bilateral arrangement of the ambulacral system, but 

 referable to an axis other than its actual antero-posterior one, is even as fully discard- 

 ed in Pourtalesia. This disposition is most plainly seen in the Spatangidte. It is as 

 follows '). If a Spatangean of any genus or species is held in supination, that, is with 

 the ventral surface upwards, and with the odd interradium backwards, and if the ten am- 

 bulacral plates of the peristome are then counted from left to right, — that is, from 

 the right side of the animal to its left, — beginning with the ambulacrum of the 

 bivium marked I; if, at the same time, in each of the ambulacra: I, II, III, IV, V, 

 thus gone över, the row of plates tirst touched is marked a, the second b, it wiJl be 

 found, that the peristomal plates I a, II a, III b, IV a, V b, are the larger ones, and 

 provided with two pedicellar pores, i. e. they are binary, bi-porous, while the peristo- 

 mal plates I b, II b, III a, IV b, V a, are smaller, and each provided with only one 

 pedicellar pore, i. e. are simple, uniporous. The identical disposition of these same 

 plates is found in the Echinoneidte, Cassidulidaä, Holastridaä, that is in all the Eden- 

 tates. In the Dentiferous Echinoids the same law is maintained, though differently 

 expressed. Among the Echinoconida; ^) it is manifested by the peristomal plates I ö, 



II a, III b, IV a, V 6, binary in the genus Echinoconus, being always larger, and succeed- 

 ed everywhere by a single plate, placed between each of them and the first triad, while 

 the peristomals I b, II b, III a, IV b, V a, are smaller, and directly contiguous to the 

 tirst triad. In the Echinida3^) its existence is obvious, and the same formula holds 

 true, masked though it be by the seemingly radiate disposition of the parts. Out of 

 the ten peristomal plates, all of them composite, the I a, II a, III b, IV a, V b, are 

 larger than the I b, II b, III a, IV h, V n. All Echinidas possess, on their buccal 

 raembrane, tive pairs of ininute free plates, each plate bearing, in the adult, a pedi- 

 cellar pore. In the very young Toxopneustes dra?bachensis, only two millimeters in 

 diameter*), out of these ten plates, those tive which correspond to the ambulacral rows 

 I a, II a, III b, IV a, V b, are larger, but still for a time are without pores, Avhile 

 the five other plates, that answer to the ambulacral rows I b, II b, III a, IV b, V a, 

 though smaller, already have received theirs. In the Cidaridas"), bi other respects so 

 widely different from the EchinidnB, out of the ten ambulacral plates surrounding the 

 oesophageal opening, those five that belong to the rows I a, II a, III b, IV a, V b, 

 exceed in size the other five that appertain to the rows I 5, II h, III a, IV b, V a, 

 while the plates of these last-named rows are, all around, the first to detach them- 

 selves from the margin of the corona, which is thus continually broken up and renew- 

 ed. Finally, in the Glypeastrida^ ''), the last arrived, in which many a primeval fea- 

 ture is vanishing or even effaced, the peristomal plates of the rows I a, II a, III b, 

 IV a, V 6, are distinguished by their superior size from those of the rows I b, II 6) 



III a, IV b, V a, the dilference, however, in a few cases having become very slight. 



Thus, it is seen, in whatever group of the Echinoidea the structure of the ambu- 

 lacra is examined, out of the ten plates composing the peristome, the five: I a, II a, 



1) Étutles, p. 13-39, pl. III— IX, XIV, XVII— XX, XXII— LII. -) Ib., p. 20. pl. XIV. 3) Ib., p. 20, pl. 

 XVII, XVIII. *) Ib., p. 27, pl. XVII, fig. 148. 5) jb., p. 28., pl. XX. *") Ib. p. 32., pl. XLIV— LII. 



