KONGL. SV. VET. AKADEMIKNfS IIANDL. BAND 19. N:0 7. 19 



this was seen, in the (JoUyritida^,, hut imperfect, the ring being open ventrally and 

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

 a feature hitherto unseen among Echinoidea. With it the appearance of a radiate 

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

 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 reflecting 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 Archteonomous, 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 endoAved with superior appliances for 

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

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

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



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

 Edentate, on its lirst 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 Echinoconida3 '), 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 therefore, the excretory opening is seen early to 

 have broken through 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 

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

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

 removed from the calyx, and they present many a feature foreign to the primary types. 



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

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

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



») Etudes, p. 79. 



