KONGL SV. VET. AKADEMIENS HANDLINGAR. BAN]\ 19. N:0 7. 37 



specimens sent me by Sii' Wyville Thomson, fragmentary as they are, has taught me 

 that, to a great extent at least, this is not the case. Ah^eady in Pourtalesia laguncula 

 Al. Ag., PL VI, Jig. 37 — 41, some important difFerences present themselves. The front 

 ambulacrum, III, retains its normal structure. The two paired ambulacra of the tri- 

 vium, II and IV, are very like those in Pourtalesia Jeftreysi, the two first plates of 

 the iirst pair being minute and wedge-shaped, and each provided with a single pore 

 the second pair M a 2, Ii b 2 and IV^ a 2, IV b 2, suddenly expanding, so as also to 

 join the aboral margins of I 6 i and I a 1, and Y a 1 and V b 1. Both ambulacra 

 continue very broad, until, dorsally, they contract rather suddenly, ending at the caly- 

 cinal system with a minute terminal plate, Pl. VII, fig 52. Thus in this species, as 

 in Pourtalesia Jeffreysi, the trivious ambulacra present throughout their whole exten- 

 sion a continuous sequence of plates, and their summits attain the calycinal system. 

 In the biviuni all this is otherwise, yet nearly in the same manner as in Pourtalesia 

 Jeffreysi. Whereas in that species the pairs of plates \ a 1 and I 6 i, as also Y a 1 

 and V b 1, have respectively coalesced into a simple compound plate, I, V, on either 

 side of the middle line, the same plates are separate in Pourtalesia laguncula, /ig. 

 ■37 — 40, the joint outline, presented by each pair, corresponding with that of the single 

 plate in Pourtalesia Jeffreysi. But while, in this latter species, the single compound 

 plate is provided with one pore only, in the former the two: I a 1 and I 6 i, as also V 

 a 1 and Y b 1, each of them retains its pore, that of I a as well as that of V 6 being single, 

 not double, as they ought to be. While in Pourtalesia Jeffreysi the I 1 and V 1 are 

 contiguous along the middle line, they are separated in Pourtalesia laguncula by the 

 wedge-shaped interradial 5 /, permitting them to come near each other at the very 

 bend only of the ventral margin of the niche, but driving them asunder by its trian- 

 gulär aboral expansion. In the same manner as in Pourtalesia Jeffreysi, these I a, b 1 

 and Y b, a 1 are Avidely separated from \ a, b 2 and V b, a 2, by the interposition of 

 the interradia 1 and 4; and the \ a 2 and V b 2, contiguous in their adoral half, 

 so as to exclude these interradials from the interradial 5 2, the sternum, then bend 

 right and left, to give room to that plate, and to the episternum, 5 a and b 3. The dispo- 

 sition of these plates thus very much resembles that seen in Pourtalesia Jeffreysi, 

 the \ a i and Y b 4 being traversed by the fasciola. In like manner, on the dor- 

 sal surface, Pl. VII, jig. 52, the bivious ambulacra are separated from one another 

 bjf the odd interradium 5, nor do they join the calycinal system, being kept widely 

 apart from it b}^ the interradia 1 and 4, meeting from either side. They terminate 

 with small, and rather sudderdy dirninished plates. 



In Pourtalesia ceratopyga Al. Ag., Pl. VII, Jig. 48 — 50, the disposition of the 

 hrst plates of the ambulacra I and V appears to be identical with that in Pourtalesia 

 laguncula, the labrum 5, 1 separating I a and V b. Each plate also has only a single 

 pore, but the row of the eight pores is not visible from the ventral aspect, as they 

 are placed on the flattened margin of the incurvation, Z^-. 49. The mode of connexion 

 of the labrum with the rest of the interradial system could not be made out, the spe- 

 cimen unfortunately being broken at the critical point. But, as stated above, there exists 

 a decided peculiarity in the condition of the interradials 1 and 4, of which, on either 



