KONGL. SV. VET. AKADEMIENS HANDLINGAR. BAND 19. N:0 7. 83 



TliG ambulacrum III witli tlie interradia 2 and 3 coiistitutc tlic shorlcned and 

 blunted front part. Tliey are of a relatively small size, normal in outlint;, adorally 

 involuted and elevated above the ventral level, and thus, contrary to what obtains 

 even in the majoritj' of the Spatangidiu, excluded from touching the ground. The 

 paired ambulacra II and IV, with the interradia 1 and 4, combine to form the leng- 

 thened middle part, the ventral surface, wholly post-oral and making nearly the half part 

 of the total length. They are expanded and more or less anomalous in outline, and 

 so is the biviuni, which, along with the odd interradium 5, makes up the abdominal 

 portion of the body. Thus far the Pourtalesiada3 upon the whole resemble the Spa- 

 tangida?, but, at the same time, present an aniount of peculiar modification unparalleled 

 araong even the most advanced of these. 



The peristome is built up solely of the tirst plates of III a b, 2 b, 3 a and 5, 

 involuted and raised above the ventral plane. It is upright, nearly vertical to the ven- 

 tral surface, and the oesophageal opening a longitudinal slit in the buccal raembrane. The 

 II and IV, having their first plates excluded from the peristome, attain the calycinal system 

 with the terminal plates of their continuously double series. The bivious ambulacra, I and V, 

 are dismembered; in each of them the first plates, notreceived into the peristome, are unitcd 

 into one single plate, and the two compound plates thus formed, by being contiguous to 

 each other and to the plates \\ a 1, 2 and IV 6 1, 2, intervene between the labrum and the 

 interradia 1 and 4, while they themselves are aborally widely separated from I, 2 and 

 V 2, which, like the following, are double, their continuous series embracing the sternum 

 and episternum, 5, 2, 3, filling with their plates \ a 4 and Y b 4 the episternal angle, 

 surrounding with I 5, 6, 7 and V ö, 6', 7 the periproctal region, and forming the 

 flanks of the abdomen, but dorsally not reaching the calycinal system. 



This breach of continuity in the ambulacra I and V, without parallel in the 

 whole class, is seen only in Pourtalcsia Jeffreysi, P. laguncula, and Spatagocystis Chal- 

 lengeri. It is brought about by the interradia 1 and 4, which close together ven- 

 trally from either side and meet again dorsally, forming, between I, 1 and V, ,i and 

 II and IV, anteriorly, and I and V posteriorly, a broad, unbroken, vertical ring all 

 around the middle of the body, a structure unexampled among Echinoidea, and, coni- 

 bined with the rudimental buccal cavity, expressive of a tendency towards an annu- 

 lose differentiation, latent, or but faintly developed elsewhere among Neonomous forms. 

 But, as said above, a few only of the species present this striking peculiarity. In 

 Pourtalesia carinata, P. ceratopyga, Echinocrepis cuneata, the interradia 1 and 4 do 

 not join ventrally in the middle, and consequently do not form a continuous ring. 

 Dorsally they meet together from either side in P. carinata and P. ceratopyga, but in 

 Echinocrepis they are separated by the ambulacra I and V, almost as in the Spatangidas. 



The heteronomy of the interradium 1, so eminently characteristic of the Spatan- 

 gidse, and particularlj;' of their låter forms, is distinctly maintained, at least in Pour- 

 talesia Jeffreysi and P. laguncula, but wholly transferred to 1 b, its formula having 

 become lb2-\-3 = 4a2: åaS. 



The obliquity indicating an imaginary discordant axis aco, IV — 1, and manifested, 

 throughout the whole class, in the disposition of the ambulacral plates of the peristome, 



