34 S. LOVEN, ON I'OURTALESIA, A GENUS OK ECHINOIDEA. 



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

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

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

 ed in Pourtalesia. This disposition is most plainly' seen in the Spatangida?. It is as 

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

 the ventral surface upwards, and Avith 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 A\ith the ambulacrum of the 

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

 thus gone over, the row of plates first touched is marked a, the second b, it will be 

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

 provided with two pedicellar pores, /. e. they are binar}-, bi-porous, while the peristo- 

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

 pedicellar pore, /. c. are simple, uniporous. The identical disposition of these same 

 plates is found in the Echinoneida;, Cassidulida;, Holastrida>, tliat 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 a, 

 II a, III b, IV a, V b, 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 6, II b. III a, IV b, V ii, are smaller, and directly contiguous to the 

 first triad. In the Ecliinidit') its existence is obvious, and the same formula holds 

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

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

 larger than the I b, II b, 111 a. IX b, V n. All Echinidaj possess, on their buccal 

 membrane, five pairs of minute free plates, each plate bearing, in the adult, a pedi- 

 cellar pore. In the verv young Toxopneustes droebachensis, only two millimeters in 

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

 1 a, II a. III li, IV a, V b, are larger, but still for a time are without pores, while 

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

 though smaller, already have received theirs. In the Cidarida'"), in other respects so 

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

 (esophageal 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 b, II b, 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 Clypeastridtp"), the last ai-rived, 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 o, V b, are distinguished by their superior size from those of the rows I b, II bt 

 III n, IV b, V a, the difference, 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) Etudes, p. 13-39, pi. Ill— IX, XIV. XVII— XX, XXII— LII. -) Ih., p. 20. pi. XIV, 3) lb., p. 20, pi. 

 XVII, XVIII. *) lb., p. 27, pi. XVII, (ii<. 148. =) lb., p. 28., pi. XX. ») lb. p. 32., pi. XLIV-LII. 



