6S 



S. LOVEN, ON POURTALESIA, A GENUS OF ECHINOIDEA. 



Holectypus maciopy 

 gus Dks. Cretac. 



Discoidea conica Des. 

 Crelac. Alb. 



Discoidea infera. 

 Des. Cretac. Turor 



stoniuus Echinids, — with the ambulacra all alike, and the peristome, central and cir- 

 cular, presenting five pairs of branchial indentations, — but exocyclic, the periproct 

 having passed from its old site at the centre of the system, into the odd interradium. 

 In consequence of this movement the costal 5 had been destroyed, but in the course 

 of time is regenerated, and even its sexual pore returns, all nearly according as the 

 periproct becomes more distant, and has been so for any length of time. Thus in the 

 oldest, Pygaster, with the periproct sub-calycinal, the 5 is completely wanting; in Pi- 

 leus, of the Middle Oolite, the periproct is dorsal, submarginal, and the costal 5 is 



restored, but destitute of sexual pore; 

 in the Oolitic species of Holectypus, 

 with the periproct marginal or ventral, 

 it is present, but imperforate, whereas 

 in the Cretaceous, in which the excre- 

 tory opening is ventral and farther dis- 

 tant from the calyx, the sexual pore 

 exists, and thus the system has once 

 more become normal. In Discoidea, 

 uf Cretaceous origin, the periproct is 

 ventral and the costal 5 present, im- 

 perforate in the lower beds, perforate in 

 the Turonian; in Echinoconus and 

 Anorthopygus the vent is posterior, 

 subventral, and the costal 5 present, but 

 without a pore. 



In like manner, among the Ateleostomes, where the excretory opening can be 

 said to have just left the calyx, as in the old Echinoneid genera Galeropygus and 

 Hyboclypus, in the Oolitic Pyrinte, the central part is irregularly broken up, or com- 

 pressed; in forms of later appearance it aborts, and the four paired costals and 

 the radials I and V close from either side. In Clypeus, the oldest of Cassidulidas, the 

 gap left by the retreating periproct is tilled by an extraordinary prolongation of the 

 almost always contiguous radials I and V, and there is hardlj^ a trace of the costal 5; 

 when the vent is removed farther back, these radials are again normal. Thus these 

 three groups on their first appearance present distinct marks of the passage of the 

 vent out of the limits of the calyx, and, while in their further development it continues 

 its receding movement, the damage it has caused is at le;ist partly repaired. 



Pygaster iiiubella 



L. Ag. Ool. Oxford 



et Corall. 



Pileiis heinispha'- 



ricuB Des. Ool. 



Corall. 



Holertypns depressiis 

 Des. Ool. Bajoc. 



Future research will perhaps afford evidence that the filtering apparatus of the 

 aqueous system primordially had its legitimate site in the central disk, and that it 

 was displaced forward through the pressure exerted by the periproct advancing from 

 the posterior interradium. Be this as it may, certain it is that, in the Mesozoic pe- 

 riod, the movement, in the opposite direction, of the excretory opening was followed 

 by the beginning of a retrograde movement of the madreporic filter. As early as 

 in Pygaster, the oldest of Exocyclic Gnathostomes, it is seen to have entered the cen- 



