KONGL. SV. VET. AKADEMIENS HANDL. BAND 19. N:0 7. 33 



b b 5, b a 5. Being thus disposed, the I a 4 and Y b 4 are traversed by the subanal 

 fasciola, and thereby brought alone to correspond to the group of ' from' two to 

 six or more ambulacral plates of I a and V b, from the sixth onwards, which, in the 

 Pryranodesmic Spatangidse, are prolonged and salient inwards, so as to fill, within the 

 subanal fasciola, the reentering episternal angle, and to determine the upward flexure of 

 these ambulacra, and which, by marking off, throughout, as ventral, the foregoing plates 

 and limiting their number everywhere to five, constitute a prominent and most signi- 

 ficant feature ^). 



The ambulacrals 1 b 4 and V a 4 of Pourtalesia Jeffreysi nearly equal in size 

 these just described, and are more regularly hexagonal. In the two succeeding pairs, 

 the fifth and sixth, the plates of I a and V b are shortened, to give place to the cir- 

 cum-anal region of the odd interradiura, while the fifth plates of I 6 and V n are 

 narrowed anteriorly in connexion with the curvature of the ambulacrum. From the 

 seventh, and onward, the following pairs have their plates nearly equal, only those of 

 the posterior rows slightly smaller, distinctly hexagonal, and slowly diminishing in size. 

 In the specimen described the fourteenth pairs are terminal. 



Accoi^ding to an almost universal rule, the ambulacra of the bivium ought to 

 ascend so as dorsally to join the calycinal system. From this the Collyritidae, of Ooli- 

 tic existence, make the sole exception hithei^to known, having their calycinal system 

 dismembered by the interjacence on either side of the unreduced interradia 1 and 4, 

 which sever from it the radial pieces I and V, and leave them in connexion with the 

 respective ambulacra. And from their erect position and convergence it follows that 

 their tops, with these radials, are brought in close contact, or so near each other, as to 

 be separated only by a iiarrow plate, which is one of a row of slender, longitudinal, 

 irregularly arranged plates stretching from the periproct to the calycinal system. At 

 a first giance this structure seems to be revived in Pourtalesia Jeffreysi, but on closer 

 inspection notable differences become apparent, Pl. I, jig. 1, 3 ; III, ftg. 11. The bi- 

 vium, far from having a vertical position, leaning backwards, so as to overhang the 

 postern slope, as in Collyrites, stretches forward longitudinally, and converges but 

 slightly, the two ambulacra being separated all along by the odd interradium 5, a pair 

 of whose plates intervenes between their summits, while the lateral interradia 1 and 4, 

 in joining from either side, combined with elements of the odd interradium, 5, keep 

 them widely apart from the calycinal system. Their summits, a pair of moderately 

 sized plates, not abruptly diminished as in the trivium, are situated at about the fore- 

 most third of the whole length of the test. Of radial (»ocular») pieces not a trace 

 is seen. 



From all this it follows that two of the fundamental and universal characteristics 

 of the Echinoidean ambulacra, their joint participation in the formation of the peri- 

 stome, and the uninterrupted sequence of their plates, are set aside in Pourtalesia. 



') Etudes, p. 15, pl. XXXII, fig. 200,— XLII, fig. 232, 



K. Sv. Vet. Akad. Handl. Band 19. N:o 7. 



