316 P. HERBERT CARPENTER. 



Curiously enough, this considerable difference in the order 

 of formation of the principal apical plates in the American 

 and European varieties of one and the same species does not 

 seem to have attracted Fewkes's attention. Had it done so, I 

 cannot but think that several passages in his memoir would 

 have been differently expressed. Thus, for example, in discuss- 

 ing the nature of the dorsocentral on p. 123, he says : " If, 

 however, in Echinoids this plate forms before the ocular and 

 genitals, and in Amphiura after the same, one is tempted to 

 ask whether they are homologous. One might, of course, 

 avoid the difficulty by the truism that the relative time of 

 development is of little consequence, and that the appearance 

 of the plate in Amphiura is simply retarded. Such an escape 

 from the difficulty does not give much satisfaction, even if we re- 

 member the abbreviated development of Ainphiura." But since 

 the under-basals (and basals ?) appear after the radial shields 

 in the West Atlantic, and before them in the Mediterranean, 

 the late appearance of the dorsocentral in Amphiura as 

 compared with the Urchins, is no argument whatever against 

 the view that this plate is homologous in the two groups ; aud 

 in fact if the relative time of appearance of the Apical Plates 

 is to be taken as a criterion of homologies, it is scarcely worth 

 while for us to attempt to arrive at a general understanding of 

 the Apical System of Echinoderms. 



Another point of the same nature is the discrepancy between 

 the descriptions given by Fewkes and Ludwig respectively of 

 the relative times of formation of the radials and terminals. 

 Ludwig 1 thinks it probable that the terminals appear before the 

 radials; while Fewkes 3 believes the reverse to be the case, 

 from the comparative sizes of the plates, though he admits 

 that he has never seen a young Amphiura "with radials 

 and without terminals." But if we may judge from the 

 analogy of the under-basals and radial shields it would appear 

 that both observers may be in the right. 



This is still more probable with regard to the development 



1 Loc. cit., p. 387. 



2 Loc. cit., p. 139. 



