6 S. LOVEN, ON POURTALESIA, A GENUS OP ECHINOIDEA. 



describcd, rugular and irrcgular. And that such and sucli alonc i.s the true and norinal 

 position uccessarily to be maintainod in every case, and to be neglected only at the 

 risk of ereating confusion, is proved beyond the possibility of a doubt by the circuin- 

 stance, that the peristomal formulär: 



I a, II a, III b, IV a, V b, 



I b, II b, III a, IV b, V a, 



in the third and tifth inembers of which a is changed into b and, invertedly, b into 

 a, and which are of universal validity in the irregular forms, hold good with equal 

 consisteney in the Endocyclic, but solely and exclusively on that one of the anibuhicra 

 which has the madreporite on its right, being recognised as the front ambulacruni, III, 

 and placed foremost, and in no other position whatever '). Then the tive ambulacra, 

 similar to one another as they apparently are, separate into the trivium, II, III, IV, 

 and the bivium. I, V, while the peristome, traversed potentially by the antero-posterior 

 axis, though in most genera it remains strictly circular or pentagonal, in others, for 

 instance in Heterocentrus and Colobocentrus, is seen to deviate slightly from the 

 regular form, and that precisely in the direction of this same axis, so as to present a 

 distinctly deeper incurvation between the two ambulacra of the bivium, without regard 

 to the direction of the longitudiual axis "). 



All this will now be perfectly clear, and it may perhaps seem little worth nien- 

 tioning that if any two different forms of Echinoids are compared together with 

 regard to their peristomal formulse, the sequence of the terms of these will invariably 

 be the same in both, provided that the counting is begun at homologous points. If, 

 on the contraxy, while in the Exocyclic Echinoid the undisputed frontal is maiutained 

 as III, in the Endocyclic any other ambulacrum except that which has the madreporite 

 on its right is tried, the formulfe will disagree all around, until that ambulacrum becömes 

 the frontal, III. Then the two formulse, the Endocyclic and the Exocyclic, will at once 

 coincide, thereb}' proving the adjustment to be true. All this is self-evident, and in 

 no way affects the significance of the formula3 relatively to the determination of the 

 antero-posterior axis. 



The normal foi^m of the single plates that compose the perisomatic and ambulacral 

 systeras is the hexagonal, which regularly manifests itself, whenever the growth of the 

 plate is not affected in consequence of its being appropriated to some special use or 

 by pressure from other plates of its oAvn system or from contiguous plates of other 

 systems. 



In regard to these general features: the distinctness of the three systems, their 

 bilateral disposition, the division of the ambulacral in a bivium and a trivium, the 

 normally hexagonal form of the plates, Pourtalesia Jeffreysi and its allies accord with 

 the other Echinoidean types and approach the Spatangidaä. 



In its general form, Pl. 1, Pourtalesia Jeffreysi is very unlike any other Echinoid 

 hitherto met with. When placed with the oral end foremost and so as to be seen 

 in its dorsal, jig. 1, or ventral aspect, fig. 2, its skeleton presents an outline which 



') Études, p. 13, 20, 27, 29, 36. -) Ib., p. 26, pl. XVIII, f. 157, 158. 



