KONGL. SV. VET. AKAnEMIENS HANDL. BAND. 19. N:0 7. 9 



very slightly from the four paired ones. In the other Adetes sharing with Anancites 

 the want of a compact sternum, the front ambulacrum is already distinct in outline 

 and in structure; thus in Hemipneustes, Holaster ^), Cardiaster. Then Prymnadete 

 genera make their appearance, in which the paired ambulacra in their dorsal petaloid 

 portions, subservient to respiration, are more or less deeply sunk, thus setting forth the 

 bilateral structure of the body, while the forepart, wliich heads its movements, assumes 

 a form of its own, allowing the front ambulacrum, with its more or less specialised, 

 often highly extensible pedicels, to sink more or less deep between the interradia 2 and 

 3. It was at the close of the Cretaceous time, and in the beginning of the Tertiary 

 period, that this peculiar independence in form of the front ambulacrum was freely 

 developed, and the older times have little comparable to the excess it attains in the 

 species of Schizaster ^), Tertiary and recent, and in the strangely resembling deep-sea 

 form, Aceste bellidifera Wyv. Thoms '). In these a more or less considerable portion 

 of the front ambulacrum is deeply sunk, so as to protrude inwardly into the peritoneal 

 cavity. But this depi'ession begins near the calycinal system, hanging down from the 

 roof, and, bulging in its middle part, continues decreasing towards the vicinity of the 

 mouth. In Pourtalesia, on the contrary, it is the ventral portion, from behind the mouth 

 and onwards, that is inverted so as to rise from the floor of the general cavity, carrying 

 along with it the peristome and the lip, and raising them into a vertical position, so 

 as to make nearly a right angle with the antero-posterior axis. 



It is from the skeleton of Pourtalesia JefPreysi that this description is taken. In 

 its principal terms it applies also to the other species of the group, to the somewhat 

 slenderer P. miranda; the more elongated P. phiale Wyv. Thoms, with its M-idely 

 gaping infra-frontal recess; the broad P. hispida Al. Ag. ; the more tumid P. laguncula 

 Al. Ag. and P. carinata Al. Ag., and even to the stout P. ceratopyga Al. Ag. with 

 its strongly expanded forepart. The two other members of the group, both apparently 

 Adete, the ovoid Spatagocystis Challengeri Al. Ag. with but a short caudal prolongation, 

 and the triangular gibbous Echinocrepis cuneata Al. Ag. with the broad forepart and 

 the subventral periproct, are linked to Pourtalesia by more than one characteristic, but 

 mainly by that most singular one, the deep infra-frontal recess. 



Thus with regard to the general form of the skeleton there is not one among all 

 the known genera of Spatangidaj, and still less among the other groups of Echinoidea, 

 to which the Pourtalesia^ bear any closer relation. In that respect, as in most others, 

 they stand alone at present. 



1) Etudes, pi. XXV, fig. 182. 



-) Comp. Schizaster antiquus Cotteau, Bull. Soc. Geol., VI, 5G7. 



') Voyage of the Challenger, 1, .376. — Ai.. Agassiz, H'p. (hall- Kchiuoidia, 195, pi. XXXII, fig. 7 — 11, 

 XXXIII a fig. 1—7, etc. 



Aka.l Hnii.ll, H.llicl HI 



