ECHINOIDEA. II. 



69 



It seems very doubtful, as pointed out by d'Arcy Thompson (Op. cit) whether the specimen 

 described and figured by Agassiz in the Challenger -Echinoidea is really the same species as the 

 P. pktale of Wyv. Thomson. The expression test very much prolonged, almost tubular dors not 

 seem so very appropriate for the form figured in the Challenger -Report, and the figure given by 

 Wyv. Thomson does not resemble the figures of the Challenger -specimen very much cither. It 

 seems, indeed, more like the Pourtalesia paradoxa described below; but Wyv. Thomson's figure and 

 description of the P. phialc are not sufficiently detailed for deciding the question, and since the type 

 specimen does not seem to exist any longer, as I am informed by Professor Bell, we must remain at 

 the decision made by Professor Agassiz and let the species described and figured as I'.phinlr in the 

 Challenger -Echinoidea keep that name. 



Some additions and corrections may be given to Professor Agassi/' description and figures 

 of the test of this species. Judging from the PI. XXII. a. Fig. 2 the odd interambulacrum is constructed 

 011 a rather different plan from what is the case in the other species of Pourtalesiae thus far known, 

 representing indeed, the most primitive structure of the plastron known among the Pourtalesiae; 

 the labruni and sternum are represented as being in contact with each other, and likewise the ambu- 

 lacra I and V are continuous, the interambulacra 1 and 4 not separating the first and the second 

 plates of these two ambulacra. This more primitive structure is the more surprising as this species 

 is otherwise a very modified form. ( )n a careful examination of the specimens in hand, I find, how- 

 ever, that the structure of the test is not as represented by Agassiz; it agrees in the main features 

 with that of the other species. |P1. YI. Figs. 1—2,7). The labrum is large and carries several primary 

 tubercles; the single plate seen on PI. XXII. a. Fig. 2 of the Challenger -Echinoidea in contact with 

 the aboral end of the labrum and which de Meijere 1 Siboga- -Echinoidea. p. 168. PL XXI. Fig.417; 

 interprets as the sternum, as it would undoubtedly have to be interpreted in case the figure were 

 correct, does not really exist. In continuation of the labrum follows a pair of large plates the ambu- 

 lacrals I. a. 2 and V. b. 2, which at their aboral eud separate a little to give room for a large, single 

 plate, the sternum, which is again followed by a pair of elongated plates, the episternal plates. The 

 two large plates following the labrum show the curious feature of being divided at their oral end by 

 a longitudinal line, which does not reach to the middle of the plate. It does not join any other line 

 and thus does not cut off any separate plate. This feature I have found quite distinct in the three 

 larger specimens examined by me (among which is one from the Antarctic Sea 1 , from the German 

 South Polar Expedition); in the two smallest specimens I have been unable to trace the limits of the 

 plates with certainty. 



Both the inner plates of the ambulacra I and Y are distinct and rather large and in confor- 

 mity with the rule: La, II. a. III. b etc.; those of the ambulacra II and IY are much smaller and seem 

 not to be always in accordance with the rule; thus in the specimen figured PL VI. Fig. 7 the plate 

 II. b was the larger — but the limits of the anterior (especially II. b and IY. a) of these small 

 plates are generally very difficult to see. The pores and tubefeet are distinct in all the 8 inner plates, 

 but there is onlv one in La. 1 and V. b. 1. The plates La. 1. b. 1 and Y. a. 1. b. 1 are in contact with 



1 In this specimen there is also at the outer end of these plates an indication of such a line; but it does not reach 

 the line from the oral end, so that the plate is not divided. 





