KONGL. SV. VET. AKADKMIKNS IIANDLINGAK. BAND 19. N:0 7. 83 



TIic ambulacrum III with the interradia 2 and 3 constitute the shortened and 

 blunted front part. They arc of a relatively small size, normal in outline, adorally 

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

 even in the majority of the Spatangida3, 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, whollj' post-oral and making neai'ly the half part 

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

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

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

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

 among even the most advanced of these. 



The peristome is built up solely of the first plates of III a b, 2 b, B 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 membrane. 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, not received into the peristome, are united 

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

 each other and to the plates II a 1, 2 and IV b 1, 2, intei'vene 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 3, 6, 7 and V 5, 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 Pourtalesia 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, ,1 and 

 II and IV, anteriorly, and I and V postei'iorly, a broad, unbroken, vertical ring all 

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

 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. cai'inata and P. ceratopyga, but in 

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



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

 gida;, and particularly of their later forms, is distinctly maintained, at least in Poui'- 

 talesia Jeffreysi and P. laguncula, but wholly transferred to 1 6, its formula having 

 become 1 b 2 -\- 3 = 4: a 2 : 4 a 3. 



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

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



