30 



S. LOVEN, ON POURTALESIA, A GKNUS OF ECllINOIDEA. 



The imaginary rostrum 

 invaginated iirolruding 



the ambulacrum III, while its sides are constituted 

 by the interradia 2 and 3, inferiorly meeting to 

 the right and left of the shortened and com- 

 pressed intra-labial space, and its innermost 

 portion, together with the elliptical membrane, 

 PL 111, jig. 12; IV, 18, ID, 20, may repre- 

 sent a pharynx, the slit being the (jesophageal 

 opening. The vaulted ymlate is armed with spi- 

 nes, stronger and more closely packed than any- 

 where, PL 1 V, fig. 24, and the site, no doubt, 

 of -A powerful ciliary activity, and outside the 

 under lip are seen the spherids in pairs or in 

 clusters, PL IV, fig. 13; VI, fig. 40, 41, 44; 

 VII, fig. 47, 48, 49, 50. Just as in another 

 part of the skeleton of certain Pourtalesiada;, as has been suggested above, a movement 

 IS introduced in the perisomatic system, directed towards an annulose conforma- 

 tion of the body, thus here also, in the peristome and surroundings, a ten- 

 dency seems to be exhibited towards a vermiform constitution. This tendency, to be 

 generally surmised already, in the Tertiary and recent Spatangida?, from the increasing 

 length and flatness of their test, the stomato-proctic axis approaching to horizontality, 

 from the reclining direction of the slender spines, and the forward movement of the peri- 

 stome, appears heightened, in another and peculiar manner, in the Pourtalesiada^ by the 

 anterior [)art of the b(jdy being definitely marked out by the terminal mouth-like re- 

 cess, and b}- a more strongly displayed differentiation of the upper dorsal side of the 

 skeleton from the under, ventral side. In the Spatangida; the frontal phyllode is still 

 in part ventral, and touches the ground in which the animal is living; in the Pour- 

 talesiadie it is elevated on the tops of the erect interradials 2 and 3, and thus brought 

 to roof dorsally tlie rudimentary mouth, therein strongly contrasting witli the other 

 four phyllodes, which all of them are ventral, and emjiloyed in forming its ground- 

 floor and lower external surface. 



In all tlie known forms of the Echinoidea, Archieonomous and Neonomous, each am- 

 bulacrum, from its beginning in the peristome to its termination, is bi-seriate, that is 

 presents an uninterrupted succession of plates, ahnost invariably arranged throughout 

 in two alternate rows. When the ambulacra of Pourtalesia Jeffreysi are examined in 

 this respect, they are found to differ more widely than those of any other Echinoid, 

 even the most specialised among Prymnodesmiie S[)atangidaj. 



The front ambulacrum, III, normal in its peristomal origin, continues so throughout 

 to its termination, /'/. //, fig. 0. It is shorter than any of the other four ambulacra. 

 It consists of thirteen to fifteen pairs of plates, out of which the second is somewhat 

 broader than the first, and the following eight or nine slowly diminish in breadth, the 

 terminal three to five suddenly becoming very minute. The first five pairs form the 

 roof of the frontal niche, or buccal cavity, PL HI, fig. 10, 12, the sixth, seventh, 



