b S. LOVEN, (IN POTIRTALESIA, A GENUS OF ECHINOIDEA. 



described, regular and irregular. And that such and such alone is the true and normal 

 position necessarily to he maintained in every case, and to be neglected only at the 

 risk of creating confusion, is proved beyond the possibility of a doubt by the circum- 

 stance, that the peristomal formuke: 



1 a, J I a, m b, [V ,1, V b, 



I /^ II t>, III rt, IV b, V a, 



in the third and lifth meud)ers of which <t is changed into b and, invertedly, h into 

 a, and which are of universal validity in the irregular forms, hold good with e(|ual 

 consistency in the Endocyclic, but solely and exclusively on that one of the ambulacra 

 whicli has the madreporite on its right, being recognised as the front ambulacrum. III, 

 and placed foremost, and in no other position whatever '). Then the five 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 tliis same axis, so as to present a 

 distinctly deeper incurvation between the two ambulacra of the bivium, Avithout regard 

 to the direction of the longitudinal axis "). 



All this will now be perfectly clear, and it may |)erhaps seem little worth men- 

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

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

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

 on the contrary, while in the Exocyclic Echinoid the undisputed frontal is maintained 

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

 on its right is tried, the formuki3 Avill disagree all around, until that ambulacrum becomes 

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

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

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

 antero-posterior axis. 



The normal form of the single plates that compose the perisoraatic and ambulacral 

 systems 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 own 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 JeftVeysi and its allies accord with 

 the other Echinoidean types and approach the Spatangidas. 



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 



Etudes, p. 13. 20, 27, 29, 36. -) lb., p. 26, pi. XVIII, f. 157, 158. 



