Abnormality in Echinus esculentus. 75 



rnality on the perignathic girdle, I am inclined to refer it to 

 the latter category. 



There is no further abnormality in interamhulacrum 3, as 

 a comparison of the tuberculatiou of the three areas (fig. 1) 

 shows. 



The most interesting feature of the specimen is the effect 

 produced by the abnormality on the perignathic girdle. 

 When the specimen was cut in half, the girdle showed the 

 features indicated in fig. 2. The two ambulacral processes 

 of the auricles bordering on iuterambulacrum 3 have coalesced 

 in the absence of the separating ridge, the joint product 

 showing traces of a median suture. The other two processes 

 of the auricles affected are practically normal, but do not 

 meet their fused fellows quite regularly, though conuected 

 to them. 



At first sight the compound " process " derived from the 

 two fused auricular elements suggests a comparison with the 

 iuterradial auricle of Cidaris. There can, however, be no 

 doubt that this resemblance is deceptive. Both sides rise 

 chiefly from the ambulacra, thus being true " processes " in 

 the sense of Duncan, as distinct from ridges. But they 

 both rest in part on the single interambulacral plate, on 

 to which they have apparently transgressed from the 

 ambulacra. 



Now in the Clypeastroida the first single interambulacral 

 plate is retained throughout life (in the absence of any 

 important peristomial resorption), and the auricle is inter- 

 radial in position. H. L. Clark has recently shown that the 

 apparently single auricle of Echinocyamus is in reality a 

 double structure, composed of two species which seem to 

 have migrated from an ambulacral position. Here, then, in 

 an area of Echinus in which a single plate is abnormally 

 retained, we find developed a perignathic structure which is 

 strictly comparable with that of the far more highly 

 specialized Clypeastroida. The abnormality is thus clearly 

 shown to be proyressive in tendency. 



It is fairly certain that Echinus is in no way ancestral to 

 the Clypeastroids, although in many respects it resembles 

 their ancestors more than they. Echinus is then, in many 

 structures, retarded in evolution when compared with the 

 accelerated features of Echinocyamus. AVe find in this 

 specimen that an accidental similarity in one structure 

 between two originally homogenetic but widely divergent 

 groups gives rise to a corresponding convergence of character 

 in another structure. This fact lends support to the view I 

 have recently put forward (Geol Mag., May 1913) that the 



