OE A SUE- ANAL PLATE BY A YOUNG ECHINOMETBA. 319 



of "Wright, " additional or sur-anal plate developed in the centre 

 of the disk before the anal opening ;" or, in the words of the more 

 recent definition of Alex. Agassiz # , it has " an ocular plate op- 

 posed to the median line of the subanal plate, the adjoining 

 genital plates uniting just in front of this imaginary median line 

 to separate the ocular plate more or less from the anal system." 



There are other points in the characters of the constituent plates 

 of the apical area that are worth a moment's 

 attention : the large size of the genital plates, 

 the small extent to which that which bears 

 the madreporite is perforated, the presence of 

 five valve-like anal plates, are indeed sugges- 

 tive points. 



But it only remains to be noticed that we 

 have here to do rather with a case of reversion 

 than of direct inheritance ; the three other young EcMnometra, 



specimens, which clearly enough belong to the enlarged 4 times, and 



, P . , . . . showing, s, sur-anal 



same species, have none ol them any persistent p i ace c ° ana i plates, 



plate. Of the three, unfortunately only one and > *», madreporic 



has its anal plates preserved, and it has but * 



three. On the other hand, if it be a case of reversion, the ancestor 



of the Echinometra had no slight resemblance to Salenia ; and the 



fact will have to be borne in mind when a serious attempt is made 



to define the relations of the Salenidae to the rest of the regular 



Echinoidea. 



The difference between this sur-anal plate and the large plate 

 on the anal area proper, which is not unfrequently found in spe- 

 cimens of Temnopleurus and other forms, seems to me to suggest 

 the question whether the sur-anal plate has really, of itself or 

 primarily, any relations to the covering plates of the anal area. 

 In the specimen here figured it has, at any rate, no share in 

 covering that orifice. Notwithstanding its retention, proper anal 

 plates have become developed, and alone exhibit relations to the 

 anal orifice. Here, just as in Salenia, there is no question as to the 

 small anal plates being distinct from the sur-anal. In Temnopleu- 

 rus and its allies the large plate lies ivithin the boundary of the 

 anus, and may even not quite touch the periphery of the anal 

 area. So far, then, as morphological identity can be spoken to by 

 similarity in position, the homology which has of late been gene- 

 rally regarded as subsisting between these plates must be some* 

 * Bull. M. 0. Z. vol. ix. p. 187. 



