36 «. LOVEN, ()N POUETALESIA, A GENUS OF ECHINOIDEA. 



Edentate forms, — contrasting with the former in the pliancy they manifest duringtheirsuc- 

 cessive appearances and vanishings, — by their progressively developed bilaterality, by the 

 variously inclined, or even horizontal stomato-proctic axis, and by the symmetry, in 

 general outline, between the two paired ambulacra of the trivium, as well as between 

 the two of the bivium. 



When Pourtalesia Jeffreysi is examined on this point, Pl. Il, Jig. 9; 1 V, 15, 16, 

 it is found at once that the two paired ambulacra of the bivium are S3'mmetrical toAvards 

 each other in their general outlines. This is manifested, as regards the two single 

 plates with which the ambulacra I and V begin, I 1 and V 1, no less by their out- 

 lines than by the condition of their pores. They are therefore in complete accordance 

 with the general rule. But, contrar}' to that same rule, the two ambulaci'a of the 

 trivium, II and IV, also present this same symmetry in the disposition of their com- 

 ponent elements. The plates II a i and IV h 1 are evidently counterparts, and so are 

 Il b 1 and IV a 1, in size, form, number and character of their pedicellar pores. And 

 yet, according to the law pervading the whole of the Echinoidea, those same plates ought 

 to differ in these respects, belonging, as they do, to different series: II a i and IV a 7 

 to the series I a ... V 6, and W h 1 and W h 1 to the series I 6 ... V a. This is 

 an essential and iraportant anomaly. It is also very remarkable that, in presence of 

 so profound a transformation, the front ambulacrum, III, alone remains intact, pre- 

 senting the normal external form and internal composition, together with the legiti- 

 mate condition of the pedicellar pores, bearing a single pore on its smaller peristomal 

 plate, III a 1, and two on the larger plate, III b 1. While, in all the rest of the Echi- 

 noidea, the composition of the peristome is expressed by the common formula: 



I a, II «., III b, IV a, V b: lar2;e, biporous; 

 I b, II b, III a, IV b, V a: small, uniporous, 



in Pourtalesia Jeffreysi the peristomal series, if arranged according to the size of the 

 plates, would be: 



I, II b, III b, IV a, V: large; 



I, II a, III a, IV b. V: small; 



and the different size of the paired trivious ambulacrals has no relation to the num- 

 ber or character of the pores. The seven plates: I, II a, II b, III a, IV a, IV b, V, 

 are all uniporous, III b alone is biporous. It is evident, therefore, that the obli- 

 quity hitherto found inherent, everywhere and without exception, to the constitution 

 of the ambulaci^al system of the Echinoidea, is evanescent in that of Pourtalesia Jeff- 

 reysi, its paired radii II and IV, I and V having their elements disposed solely with 

 relation to the antero-posterior axis of the body, and not to a subordinate deviating 

 axis cco}, as in all other Echinoidea. The elsewhere all-pervading obliquity is on 

 the point of disappearing, but a trace of it is still left in the front radius, III, 

 which has, as normal, its a plate uniporous, and its b plate biporous. 



NoAV, bearing in mind the unvarying constancy of the ambulacral characteristics 

 in all the numerous genera and species so highly diversified in other respects, with 

 which we are familiar, it seems that we might be alloAved to expect their contrasts, so 

 strikingly set forth in Pourtalesia Jeffreysi, to possess an equally pervading validity 

 as characteristic of the entire genus and its allies. But a careful examination of the 



