ECHINOIDEA. II. 



69 



It seems very doubtfnl, as pointed out b}- d'Arcy Thompson (Op. cit.) whether the specimen 

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

 P. pJiialr of Wyv. Thomson. The e.xpression test very much prolonged, alniost tubular does not 

 seem so ver\' 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 either. It 

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

 description of the /-". pliialc are not snfficiently detailed for deciding the question, and since the type 

 specimen does not seem to exist ^xw longer, as I am informed by Professor Bell, we mnst remain at 

 the decision made b\- Professor Agassiz and let the species described and figured as P.phiaL' in the 

 Challenger -Echinoidea keep that name. 



Some additions and corrections mav be given to Professor Agassiz' description and figures 

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

 on a rather different plan from what is the case in the other species of Pourtalesiæ thns far known, 

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

 the labrum and sternnm are represented as being in contact with each other, and likewise the anibu- 

 lacra I and \' are continuous, the interambiilacra i and 4 not separating the first and the second 

 piates of tliese two ambnlacra. This more primitive structure is the more surprising as this species 

 is otherwise a very modified form. On a careful examiliation of the specimens in hånd, I find, how- 

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

 with that of the other species. (PI. VI. Figs. i —2, 7). The labrum is large and Carries several primary 

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

 the aboral end of the labrum and which de Meijere ( Siboga»-Echinoidea. p. 168. PI. XXI. Fig. 417) 

 interprets as the sternnm, as it woiild undoubtedly have to be interpreted in case the figure were 

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

 lacrals I. a. 2 and \'. b. 2, which at tlieir aboral end separate a little to give room for a large, single 

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

 two large piates following the labrum show the curious feature of being divided at their oral end b\' 

 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 fonnd quite distinct in the three 

 larger specimens exaniined b\- me (among which is one from the Antarctic Sea', from the Oerman 

 vSouth Polar Expedition); in the two smallest specimens I ha\-e been unable to trace the limits of the 

 piates with certainty. 



Both the inner piates of the ambnlacra I and V are distinct and rather large and in confor- 

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

 not to be alwa\s in accordance with the rule; thns in the specimen figured PI. VI. Fig. 7 the plate 

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

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

 but there is only one in I. a. i and V. b. i. The piates I. a. i. b. i and V. a. i. b. i are in contact with 



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

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



