KONGL. SV. VET. AKADEMIENS HANDL. BAND. 19. N:0 7. 31 



eighth and ninth curve upwards, and constitute Avith the last five or six the mesial, 

 moderately impressed part of the blunted front, the small terminal plates closing 

 with the calycinal system just on reaching the dorsal surface. 



The two paired ambulacra of the trivium, II and IV, PL I J, fiy. 9; I, 1, 2, 3; 

 III, 10, 11, are souicAvliat larger than the front one. They are slightly curving for- 

 ward symmetrically, })road in the middle and suddenly contracted at their summits. 

 In the described specimen the number of plates is about fourteen in each. The first 

 two are, as above stated, small and wedge-shaped, the second pairs suddenly expand, 

 and still more so the following pairs, each ambulacrum attaining its greatest breadth 

 before reaching half the height of the test, after which it slowly decreases, the plates 

 of the posterior row in each, the II a and IV b, surpassing throughout those of the 

 anterior, and both nearly preserving the hexagonal form. The thirteenth and four- 

 teenth pairs suddenly contract, and become very minute, when joining the calycinal 

 system. 



Thus the trivious ambulacra, from their beginnings in or near the peristome, 

 conformably to the general rule, present throughout their whole length a continuous 

 double row of plates, and their summits attain tlie calycinal system. In the bivium 

 all this is different. 



As above stated, the ventral surface, PL I, fig. 2, 8; 11, 9; IV, 15, presents an- 

 teriorly two contiguous elongated plates, I, 1, V, /, one on each side of the mesial 

 line, having the terminal portions of their gradually narrowing ndoral halves bent over 

 into the infra-frontal niche, PL IV, fiij. 16. Their anomalous form and their contiguity 

 to the first plates of II a and IV b combine to make it more than uncertain how to 

 distribute correctly, to their respective ambulacra, all these six closely joining plates, 

 from external inspection alone. But their relations at once become obvious, when they 

 are examined from the peritoneal cavity, after the test has been cut open longitudi- 

 nally, PL 111, fig. 10, 11: IV, 18, 19. In the whole of the Echinoidea the neural collar, 

 at each of its five angles, sends off a cord that follows the mesial line of each ambu- 

 lacrum on its inner surface, and gives off alternate branches which enter the pedi- 

 cellar pores, and come out again on the outside of the test, there to distribute them- 

 selves among the different external organs,'). The neural collar and the main nerve- 

 cords are closely accom])anied by the circular vessel and the great trunks of the 

 aquiferous sj^stem, outside emerging along with tliem from out of the pores, and entering 

 the tubes of the pedicels. When the test of Pourtalesia Jeffreysi is laid open, and the 

 recess is examined on its peritoneal surface, PL 111, fig. 10, 11; IV, 18, 19, it is seen 

 that here also the neural collar and the circular aquiferous vessel respectively send 

 out their five main branches, one for each ambulacrum. Of these an odd one runs 

 along the mesial suture of the vaulted front ambulacrum. III, on the top of its roof, 

 and ascends to the calycinal system, giving off, on every plate, a branch to its pore, 

 but two on the plate III b 1, fig. 10, 18, 19. The two paired ambulacra II and TV also 

 each receive two main trunks, one neural, one aquiferous, giving off' branches to the 



>) Etudes, p. 8, pi. II, fip. 28—31. 



