KONGL. SV. VET. AKADEMIENS HANPL. BAND 19. N:(i 7. 33 



5 b 0, b a 3. Being thus disposed, the I a 4 and \ b 4 are traversed by the subana] 

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

 six or more ambulacra! plates of I <t and V b, from the sixth onAvards, Avhich, in the 

 Pryranodesmic Spatangidaj, are prolonged and salient inAvards, so as to fill, Avithin the 

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

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

 and limiting their number CA'eryAvhere to five, constitute a prominent and most signi- 

 ficant feature ^). 



The ambulacrals I b 4 and V a 4 of Pourtalesia JcftVeysi nearlj' equal in size 

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

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

 cura-anal region of the odd interradiura, Avhile the fifth plates of 1 b and V a are 

 iiarroAved anteriorly in connexion Avith the curvature of the ambulacrum. From the 

 seventh, and ouAvard, the folloAving pairs have their plates nearly equal, only those of 

 the posterior roAvs slightly smaller, distinctly hexagonal, and sloAvly diminishing in size. 

 In the specimen described the fourteenth pairs are terminal. 



According to an almost universal rule, tlie ambulacra of the bivium ought to 

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

 tic existence, make the sole exception hitherto knoAvn, having their calycinal system 

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

 Avhich sever from it the radial pieces I and V, and leave them in connexion Avith the 

 respective ambulacra. And from their erect position and convergence it folloAA's that 

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

 be separated only by a narroAv plate, Avhich is one of a roAv of slender, longitudinal, 

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

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

 inspection notable differences become apparent, PL 1, fig. 1, 3 ; 111, fig. 11. The bi- 

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

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

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

 of Avhose plates intervenes between their summits, Avhile the lateral interradia 1 and 4, 

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

 them Avidely 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 Avhole length of the test. Of radial (»ocular») pieces not a trace 

 is seen. 



From all this it foUoAvs that tAvo of the fundamental and uniAersal 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. 16, pi. XXXir, fi?. 200,— XLII, <ns. 232. 



K. Sv. Vet. jMtad. Handl. Band 19. N:o T. 



