56 PROP. p. M. DTJiirCA.]S" AND MR. W. P. SLADEN OIT 



Radial Plates. — These are upon tlie same plan as those of 

 Ccelopleurus, aad as the peculiar central ridge and the position 

 of the two optic pores have been described by Loven (op. cit. 

 pp. 66 and 67), it is not necessary to do more than refer to that 

 careful naturalist's descriptions. 



XII. Comparison hetween the two Genera. 



It is evident from the descriptions given of the ambulacra, 

 radial plates, and interradia of the fossil and recent species of 

 the genus Goelopleurus and of the species of Arhacia, that these 

 forms have a great similarity of structure. In all, with the ex- 

 ception of Arhacia nigra, the compound plates of the ambulacra 

 are formed of an adoral and an aboral demi-plate with a large 

 central primary plate. 



In all the forms the optic pores are double, and the per- 

 foration is in the adoral edge of the plate, a process separating 

 the pores. In all the forms the median or vertical sutures of 

 the interradia are marked with knobs or ridges, which corre- 

 spond with sockets or short grooves on the opposed plate-edges. 

 This kind of dowelliug is even seen in the ambulacra of Arhacia 

 and along the transverse interradial sutural edges of Goelopleurus. 



Arhacia nigra is an exceptional species, and, as will be shown 

 in Part II. of this communication, belongs to a different genus 

 from the Arhacia. 



It is of importance to remember that Goelopleurus was the 

 first of the two genera, and that there were species with the 

 peculiar ambulacra in the Eocene, Oligocene, and Miocene ; that 

 the recent species from the Indian seas only differs from the 

 Miocene form in having high and not oblique interradial plates ; 

 and that all the species of Arhacia which were examined, except 

 one which other authors have eliminated, present no greater dif- 

 ferences than can be accounted for on the theory of descent. 

 The Arhacia are recent forms. 



DESCEIPTION OF THE PLATES. 



Plate I. 



Fig. 1. Strongylocentrotus droebachiensis. A compound ambulacral plate from 

 a young example, a', the aboral primary poriferous plate ; a", the 

 adoral primary ; b, the intermediate demi-plate. Magnified. 

 2. Goelopleurus Pratti. Compound ambulacral plates and adjacent inter- 

 radial plates from the region of the ambitus. Magnified. 



