i \i IMOCRINUS DIOMEDiE. 83 



recalling the arrangement of the Arbaciadaa, Biirrounded however in ii < 1 < 1 i - 

 tion bv two other concentric rings of smaller plates; while in Neolampas 

 we have an anal pyramid composed at firsl of five plates, and subsequently 

 of eighl (Figs. 7. 8). The number of plates of the anal pyramid of Palae- 

 ostoma also varies from five * to seven.f 



Of course there is the other and former point, of view, thai in the earlier 

 types of Echini (Palsechinus and the like) the young also possessed a single 

 suranal plate covering the anal system, and that this was transformed very 

 early into a number of plates forming two or more concentric rings around 

 the anal system. But the history of the plates of young Cidaris, one of 

 the earliest of the Echinidfe to appear belonging to a type which has 

 been most persistent from the Jura, and which still forms in our seas an 

 important element in the eehinoidal fauna, is decidedly opposed to this 

 view. The young Cidaris has, as we have shown, five (infrabasals) radial 

 plates, which divide laterally and form a first ring of ten or more plates 

 adjoining the genital ring. We are therefore justified in assuming that 

 in the Palaaehinidae there may have been in the young a similar radial 

 system of five plates, and these would naturally form the structure which 

 has been preserved in their anal system ; so that we should be far more 

 inclined to look upon the plates enclosed within the genital ring, such 

 as are found in joung Cidaris, as representing the primordial type of 

 the Palsechinidae, and that this type had gradually in geological time 

 been modified so as to leave in Acrosalenia only three of the original 

 five plates, in Salenia only one, exclusive of the smaller plates subse- 

 quently formed in the inner area enclosed by the five original plates. 

 This degeneracy, so to speak, would account for the asymmetrical posi- 

 tion of the plates in Acrosalenia and Salenia, as well as in the young 

 of so many Echinida? ; so that we should no longer consider the apical 

 system of Salenia and its representative at the present day in the apical 

 systems of young stages of many Echini as reproducing the typical or 

 original type of abactinal system from which all others have been derived. 

 The present explanation of the homologies of the plates of the young 

 Echinidae and of Salenia has the advantage of being consistent with the 

 palrcontological development, while the former one has not.:}: 



* A. Agassiz, Revision of the Echini, Plate XXXIII. Fig. 14. 

 t Loven, Pourtalesia, Plate XVI. Fig. 191. 



J Xeumayer appears justified in denying that the apical system of Salenia represents the typical 

 stock from which the apical system of the recent Echinidae has been derived. 



