KONGL. SV. VET. AKADEMIENS HANDLINGAR. BAND. 19. N:0 7. 69 



tral part of the calycinal system, effacing the suture which limits the costal 2, and such is 

 its condition in all known Echinoconidfe^), except that in Discoidea it is seen to spread 

 to all the five costals"). When the Ateleostomata make their first appearance, as Echino- 

 neids, the madreporite is restricted to the costal 2; it is 

 more expanded centrally in the Cretaceous Pyrin* than in 

 the Oolitic; in the recent Echinoneus^) it is central, and 

 the sutures effaced. In some few of the oldest of Cassidu- 

 lidae, in which the centre is fractured into irregular detached 

 pieces, it is limited to- the costal 2; in the great majority of 



species it occupies the centre, expanding it largely. In the Pyrina Guerangeri Pyrina Durandi 



recent Cassidulus all the sutures are obliterated. ^°Bajo°°''''' ^" ^Tto^n'*'"' 



The growing deviation from the original Gnathosto- 

 mous type reaches its highest point in the Spatangida3. Introduced by the Collyrites, 

 of Oolitic existence, they make their appearance early in Cretaceous time as Holastrida3. 

 Deeply contrasting with the Orthoproctic plan of construction: the circular ambitus; 

 the central and circular peristome not modified during growth, and formed out of 

 ambulacral and interradial plates of both series, at equal parts in either; the seemingly 

 radiate disposition of the areas, ambulacral and interradial; the specious homocentri- 

 city of the spines; the general uniformity of the pedicels, only exceptionally or in 

 part branchial; — all features present already in the ancient Cidaridae, consistently 

 maintained up to the present era by them and by numerous Echinidae, and but slightly, 

 if at all, modified in a few forms^) — , the evolution of the Spatangidge is pervaded 

 by an increased and largely diversified tendency toAvards the differentiation of the 

 skeletal elements into modifications unseen before, only in part begun by the Echino- 

 neid*. and the Cassidulida^. The gradual lengthening of the whole framework, the 

 forward movement of the trivium, the increasing growth and differentiation of the 

 abdomen and of the bivium, more and more overcome the inherited globosity; the 

 alimentary canal extends horizontally, its oesophageal opening advancing, while the 

 excretory aperture retrogrades; the bilateral symmetry becomes apparent, betraying a 

 dawning approximation towards a vermiform disposition; the trivium and the bi- 

 vium are more distinctly constituted: in the former the ambulacrum III is specialised 

 as the front ambulacrum, and provided with pedicels of a peculiar structure, in the 

 later genera at least not subservient to respiration, while the II and IV are paired 

 and made counterparts in outline, though still unsymmetrical with regard to their 

 constituent plates, and the I and V are made symmetrical in^vardly as well as in their 

 outlines; the peristome, consisting of biseriate ambulacrals and single interradials, 

 is originally pentagonal, but nearly always becomes modified during growth; of 

 the perisome, the four lateral interradial areas are distinctly disposed into two pairs, 

 the symmetry of the anterior pair, 2 and 3, being perfect, while that of the posterior, 

 1 and 4, is qualified, in 1 a, or in 1 a and 1 b, by an heteronomous disposition un- 



') See woodcut on the preceding page. -) Etudes, p. 81, pi. XV, fig. 133. *) lb. pi. XV, fig. 131. 

 *) lb. p. 26, pi. XVIII, fig. 153—158. 



