16 S. LOVEN, ON POXJRTALESIA, A GENUS OF ECHINOIDEA. 



fig. 2; II, 9; III, 10. But in Pourtalesia tliis essential constituent of the ventral surface 

 is not composed of two collateral, lengthwise suturally uiiited plates, but compact, all 

 of one single piece, shield-like, slightly bulging, pentagonal, ^videning a little anteriorly, 

 and having its front margin faintly arcuated. Although, if taken as a whole, surpassing 

 in size all the other plates, it is considerably smaller, relatively to the entire ventral 

 surface, than the sternum of the Spatangidaj, but notwithstanding this and its backward 

 position, it is at once recognised by its mesial position and its numerous crowded 

 spines. It is followed by a pair of Avell defined and fairly sized episternals, somewhat 

 lengthened, but else not unlike those of Meoma and Brissus ^), crossed, as they ought 

 to be, b}' the ventral portion of the subanal fasciola, and contracted a little posteriorly 

 so as to form on either side, with the first pair of abdominals, 5 a 4, 5 b 4, &w open 

 but distinct episternal angle into which are received the bivious ambulacrals \ a 4 and 

 Y b 4, Pl. II, fig. 9. By curving upward rather abruptly, Pl. I, fig. 3, this first pair 

 of abdominals and of pre-anals constitute the hindermost blunted end of the test, and 

 cause the second and third pre-anals, 5, 5 and 5, 6, to form the dorsal, flat or even 

 slightly concave surface of the caudal prolongation. Of these the first-named, 5, 5, is 

 traversed by the fasciola, Pl. I, fig. 1, 3 ; II, 9. Then come three pairs of anal plates, 

 5, 7, 5, 8, 5, 9, Pl. I, fig. 4, diverging sideways to give room to the excretory opening, 

 and having their inner margins cut out for the periproct, the two first-named forming 

 its inward sloping sides, Pl. I, fi.g. 4; III, 13. The third, 5, 9, turns abruptly över 

 on the back, and is there followed by a forwards directed series of seven pairs of 

 dorsal plates, 5,io to 5,ic which gradually contract, Pl. I, fig. 1; III, 11. They are at 

 first transverse, then somewhat squarish, at last longitudinal, all irregularly hexagonal, 

 having one of their sides lengthened, and another hinder one very short, thus giving 

 rise to an almost straight-looking middle suture. It is this part of the back which 

 is raised into a distinct mesial ridge subsiding into the simple convexity of the pairs 

 5, 15 and 5, 16. In these two pairs the a plate becomes distinctly smaller than the b 

 plate, in a manner analogous to the checked growth of the corresponding plates 

 observable in certain Spatangida3 ^). Situated within the anterior third part of the dorsal 

 surface, and separating from one another the terminals of the bivious ambulacra, the 

 dorsals 16 of the interradium 5 are there met by the two lateral interradia 1 and 4, 

 PL. I, fig. 1; III, 11, joining one another from either side, and intervening with their 

 large plates between them and the calycinal system. But it is only at the first colli- 

 sion that the dorsals give way. Taking advantage, as it were, of the intersecting 

 point of the sutures of the four terminal plates of 1 and 4, the odd interradium 5 

 there reappears as 5, 17, rather out of shape, and with the a plate still more reduced, 

 Pl. I, fig. 1; II, 9, or altogether löst in the conflict, Pl. III, fig. 11. And when, on 

 the further side of \ b 10 and 4 a 10, it comes forth again, its two plates, 5, 18, have 

 resumed their proper size and due proportions, and at the same time have attained 

 their legitimate position in close contact with the calycinal system. Thus, in Pourtalesia 

 Jeffreysi, the sequence of these dorsal plates of the interradium 5 is broken. From 



•) Études, pl. XXXIV, XXXV. 2) xi,._ p. 60. 



