KONGL. SV. VET. AKADEMIENS HANDL. BAND 19. N:0 7. 19 



this was seeii, in tlie CollyritidaB, but imperfect, the ring being open ventrally and 

 closed dorsally only. In these Pourtalesia^ it is complete above as below. This is 

 a feature hitberto unseen ainong Echinoidea. With it the appearance of a radiate 

 disposition of the skeletal elements, still kept up, to no small extent, in the Spatangidte, 

 is destroyed in an essential degree, and a tendency betrays itself towards an annulose 

 differentiation of the bilaterally symmetrical constituents of the cylindroid skeleton. 

 When retiecting on the great difference between Tiarechinus and Cidaris on the one 

 hand, represented already in the earliest of Mesozoic times, and Pourtalesia on the 

 other, nowhere recognised among extinct forms, and on the general character of the 

 succession of varying forms separating their epochs, the idea presents itself that, 

 while one branch of the Echinoidea, the Archfeonomous, has tenaciously persisted in 

 maintaining its original features, the other, the Neonomous, has been striving all the 

 time to lay aside the radiate sameness of the ancient structure, and gradually to 

 approach a higher standard of organisation endowed with snperior appliances for 

 ministering to the varied activities of life; and that something like siich a stage of 

 its evolution is on the eve of being touched, among the Pourtalesiadaj, by means of this 

 primitive attempt at an annulose difierentiation of some of the skeletal elements. 



Each of the different groups of Exocyclic Echinoids, Dentiferous as well as 

 Edentate, on its iirst appearance, has the excretory opening placed at or near the calycinal 

 system, and from thence, in the course of time, it gradually recedes farther and farther 

 back. Thus, among the Echinoconida; '), in Pygaster, the oldest of them, it is dorsal 

 and still partly calycinal, as though it had just broken through the circle of costals; 

 in' Pileus, of the Middle Oolite, it is dorsal, sub-marginal; in the Oolitic species of 

 Holectypus it is marginal or ventral, in the Cretaceous ventral and farther removed; 

 in Discoidea, of Cretaceous origin, it is ventral, in Echinoconus and Anorthopygus 

 posterior, sub-ventral. In this group thei-efore, the excretory opening is seen early to 

 have broken thi-ough the calycinal circle, and, when out of it, to retrograde farther and 

 farther back. But this branchlet of the Dentiferous type was not strong enough to 

 endure all through the Tertiary period and up to the present era, — its sole survivor, 

 the Pygaster relictus of the Caribbean sea, is of very diminutive dimensions — , it is 

 upon the main branch, the Endocyclic, that it has devolved to people the seas of 

 sQccessive geological ages, and at its side the Dentiferous Exocyclics are actually repre- 

 sented by the Clypeastridaä, of all but Tertiary origin. In these the periproct is far 

 removed from the cal3'x, and they present many a feature foreign to the primary types. 



Long before their appearance the other great branch of the Echinoidea, the 

 Edentate, not upon record from Palasozoic time, already formed a conspicuous part of 

 one of the earliest among Mesozoic faunas as yet recognised. From whatever point 

 of the common trunk it may have once originated, whatever may have been its phases 

 of existence previous to its appearance, and still unknown to science, it gives evidence 

 enough of having undergone profound modification, and, in assuming new distinctive 

 features, of having done away with structural formulas deeply characteristic of its elder 



>) Études, p. 79. 



