32 S. LOVEN, ON POURTALESIA, A GENUS OF ECHINOTDEA. 



pores of the plates II a 1, II a 2, etc. and II h 1, II b 2, etc, on the right side, 

 as also, on the left, to those of the plates IV b 1, IV b 2, IV b 3, etc, and IV a 1, 

 IV a 5 etc. And from these beginnings the main trunks ave seen to einit regularly 

 alternate branches to the ambulacral pores, Pl. Ill, fig. 10, 11. It follows from this 

 that these plates were rightly referred, from the outside aspect, to the ambulacra II 

 and IV, as true peristomal plates, although excluded from their legitimate position. 



Lastly, the neural collar and the circular vessel each give ofi' two main branches 

 directed backward and slowly diverging, Pl. IV, fig. 18, 19; III, 10. Just on their 

 leaviug the collar, a first branchlet descends from each of them to the pedicellar pore 

 of one of the two single contiguous plates V and I, fig. 18, V, 1, I, /; fig. 1,9, V, 1, 

 thereby rendering it manifest that these plates are in reality what they were pro- 

 nounced to be from without, the reduced and displaced peristomal plates of the 

 bivium. V, 1 and I, 1. But when the further continuation from thence of the bivious 

 ambulacra is looked for from Avithout, Pl. 1, fig. 2, the sequence of their plates is 

 found to be broken by the interposition of four longitudinal plates of considerable 

 size evidently not ambulacral, the less so, as none of them presents] any trace of a pedi- 

 cellar pore. They belong, as shown above, to the perisome,being the \bl,\b2 + 3 and 

 4 a i, 4 a 2, of the interradial system. Viewed from the peritoneal cavity, Pl. III, 

 fig. 10, these same plates are seen to be spanned by the two trunks which, after their 

 first branchlets, do not give off a second till reaching, on the farther side, two 

 pairs of plates, contiguous in the mesial line, 1 a 2, I b 2, Y b 2, Y a 2. Thei"e 

 again a branchlet is seen to proceed from each trunk, the first for the inner of 

 the two plates: I a 2, Y b 2, the second for the outer ones: I b 2, Y a 2, and so 

 on alternately and continuously to the summit of the ambulacrum, Pl. III, fig. 



II, 14. In Poui"talesia Jeffreysi, therefore, not only the first plates of the bivium 

 are single, not double, but they are, moreover, widely separated from the next in 

 ordei', and the two ambulacra begin, as it were, anew, at about the posterior half of 

 the body, each with the regular double alternate series of plates. But there, as where 

 they commenced withthe two first single plates I, 1 and V, /, the two ambulacra are again 

 contiguous in the mesial line, the \ a 2 and Y b 2 touching each other by their fore- 

 pai'ts for more than the anterior half of their inner margins, thereby excluding the 

 interradials 1 and 4 from the posterior interradium 5. After that their hind portions 

 separate, bending right and left, to give place to the sternum, Pl. I, fig. 2; II., 9; 



III, 10. The I b 2 and V a 2 are shorter, less irregular, but sub-pentagonal, slightly 

 curved, and someAvhat broader in front. Of the third pair the 1 a 3 and Y b 3 are 

 elongated, irregulai"ly hexagonal and contiguous to the sternum and to the forepart of 

 the episternum, while the I b 3 and Y a 3 are broader, but much shorter, thus 

 allowing the ambulacrum to bend upward at a right angle, Pl. I, fig. 3. Hence the 

 fourth pairs lie at the basis of the ascending portions of the bivium. The I a 4 and 

 Y b 4 are hexagonal, Avith their hind-portions Avidening and extending beyond the 

 posterior limits of the ambulacrum, so as to börder inwardly upon the episternum 5 

 b 3 and 5 a 3, then to fill out on either side the episternal angle, and to join the 

 whole length of the first pre-anals 5 b 4, 5 a 4, and the greater part of the second, 



