30 



S. LOVEN, ON POURTALESIA, A GENUS OF ECIIINOIDEA. 



The imaginary rostrum 

 invaginated protniding. 



the ambulacrum III, while its sides are constituted 

 by the interradia 2 and 3, inferiorly mecting to 

 the right and left of the shortened and com- 

 pressed intra-labial space, and its innerraost 

 portion, together with the elliptical membrans, 

 Pl. 111, fig. 12; IV, 18, 19, 20, may repre- 

 sent a pharynx, the slit being the oesophageal 

 opening. The vaulted palate is armed with spi- 

 nes, stronger and more closely packed thaii any- 

 where, FL IV, 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 

 dusters, Pl. IV, fig. 15; VI, fig. 40, 41, 44; 

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

 part of the skeleton of certain Pourtalesiadae, 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 surrouiidings, a ten- 

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

 generally surmised already, in the Tertiary and recent Spatangida3, from the inci-easing 

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

 fi-om the reclining dix-ection of the slender spines, and the forward movement of the peri- 

 stome, appears heightened, in another and peculiar manner. in the Pourtalesiadai by the 

 anterior part of the body being definitely marked out by the terminal mouth-like re- 

 cess, and b}' a more strongly displayed differentiatioii 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- 

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

 to roof dorsally the rudimentary mouth, therein strongly contrasting with the other 

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

 floor and lower external surface. 



In all the known forras of the Echinoidea, Archaäonomous 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, almost 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 Piymnodesmiic Spatangidse. 



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

 to its termination, Pl. II, fig. 9. 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 tive suddenly becoming very minute. The first tive pairs form the 

 roof of the frontal niche, or buccal cavity, Pl. III, fig. 10, 12, the sixth, seventh, 



