82 S. LOVEN, ON POURTALESIA, A GENUS OF ECHINOIDEA. 



V. THE POURTALESIAD^. 



Tlie cbavacteristics of tlieir skeletoii. Tliey constitute a distinct faraily equivaleiit to tliat of tlie Spa- 

 tangidit! aud the Cassidiilid;ie. Tlieir geographical and batliyraetrical distribution. 



Eight species comprised in the genus Pourtalesia, one Echinocrepis and one 

 Spatagocystis. make the whole of what is known at present of the little group discuss- 

 ed in the foregoing pages with regard to its skeletal morphology. It remains to 

 exposé its characteristics in a comprehensive form, and to determine its systematic 

 position. In this attempt I feel all the inconvenience of being able to speak, from 

 immediate observation, of but a single species in an entire condition, of three others 

 in a fragmentary state only, and of having had before me no specimen at all of Spatago- 

 cystis. It so happens, moreover, that Pourtalesia Jeffreysi, the only species examined 

 with some degree of completeness, appears to be, in certain respects, of a more advan- 

 ced character than the rest, and therefore, as being less in harmony with their mode 

 of conformation, perhaps not rightly to be regarded as embodying the typical features 

 of the group. However, notwithstanding these shortcomings, and with the reservation 

 therein implied, and calling to mind that this is not the first occasion, nor will be 

 the last, when a species that chances to be the most familiar to us, is put forward 

 as the type of its kind, I venture on the following description, with Pourtalesia Jeff- 

 reysi in the foreground. 



The general form of the skeleton of the Pourtalesiadse is more lengthened than 

 that of most other Neonomous Echinoids, sub-cylindroid or ob-conical; anteriorly more 

 or less truncate, soraetimes broad; in the middle slightly tumid; posteriorly tapering; 

 dorsally convex, sometimes even raised into a hump; ventrally rather flat. Below 

 the slightly overhanging front the forepart is invaginated into the peritoneal cavity, 

 so as to form a deep infra-frontal recess, a rudimentary mouth and buccal cavity, open- 

 ing anteriorly and ventrally, having at its bottom the peristome, — a disposition 

 unexampled anywhere else in the whole class, an incipient feature approximating to 

 what obtains in worms. Posteriorly the body, in most of the species, terminates in a 

 caudal prolongation, and then the periproct is subdorsal, as among the Cassidulida?, 

 or at least posterior; in one species, Echinocrepis cuneata, the body is simply pointed 

 behind and the peripi'oct subventral. The stomatoproctic axis is nearly parallel to 

 the ventral plane. 



The bilateral symmetry of the constituent elements is highly developed, while, at 

 the same time, the dorsal side contrasts with the ventral in a strongly marked man- 

 ner. In these points the Pourtalesiadas depart from the Archajonomous type more 

 widely than any other group. 



The three skeletal systems, forming together an unbroken surface, are all pre- 

 sent, in different states of development: the perisomatic system predominates; the 

 ambulacral presents its five rays, while the calycinal system is seen to lose its character, 

 and to verge upon decay. 



