122 Prof. P. M. Duncan and Mr. W. P. 81aden on 



required, the anal orifice. In tlie irregular Ecliinoidea the 

 plates above the anus are often the largest. 



II. On the 6^e/iera o/'Saleniida3. 

 Genus Acros alexia, Agass. 



The structures of the apical system of some of the species 

 of this genus are more readily comparable with those of the 

 other great groups of the Ecliinoidea than those of Salenia. 

 Thus Acrosalenia spinosa, Agass., from the Inferior Oolite, 

 presents a symmetrical pentagonal dorso-central plate, which 

 is in the polar axis of the test and in the antero-posterior 

 axis of the apical system ; it is placed within the antero-lateral 

 basals, to which it is fixed by suture. It is limited poste- 

 riorly by the periproct, which curves its posterior border. 

 The plate is ornamented with small tubercles which carried 

 spines, its construction resembles that of the basals, and 

 it was immovable. The periproct is thrown backwards, and 

 owing to the fixity of the dorso-central plate the posterior 

 basal (5) became small and the radials I. and V. came within 

 the periproctal ring. The anal (periproctal) plates are not 

 preserved, but they doubtless surrounded the anal aperture 

 within the periproct. 



In Acrosalenia decorata (Haime, sp.), Wright, from the 

 Corallian, the dorso-central plate is in its normal position, and 

 occasionally it is perfectly hexagonal and in no way eroded 

 for the periproct (Wright, Monogr. Brit. Foss. Echin. Oolitic 

 Form., Pal. Soc. 1856, pi. xvii. fig. 1/). Behind this plate 

 are five or six others, and then comes what might be w-ell 

 considered to be the periproctal ring ; but from the position 

 of the radial plates it would appear that the smaller plates in 

 contact with those behind the dorso-central plate may be 

 within the periproct. Almost as good an instance of the 

 preservation of a geometrical dorso-central plate without any 

 notching for the periproct is seen in Bone's good figure on 

 the same plate, fig. 1 g^ and there are no supplementary plates, 

 but a large periproct. Of the independence of the dorso- 

 central in this species there can be no doubt. 



Acrosalenia Loweana^ Wright, has small additional plates 

 which separate the dorso-central from the postero-lateral basals, 

 but it would seem that they are really supplementary plates. 

 In Acrosalenia Wiltoni, Wright, there is a row of plates 

 between the dorso-central and the large periproct, and they 

 are supplementary plates (Wright, op. cit. pi. xvii. fig. 5). 



The entrance of the posterior radial plates into the peri- 



