THE FAMILY AKBACIAD^. 53 



corresponds numerically with that in A. stellata and A. punctu- 

 lata, several of the preceding series form distinct incipient 

 triplets. 



It is especially interesting to note how closely the two species 

 Arhacia stellata and A. punctulata resemble (Jcelopleurus in 

 their ambulacral structure, even to the trapezoid form of the 

 small or demi poriferous plates ; and the significance of the fact 

 is further enhanced when it is remembered that these two species 

 o? Arhacia approach Coelopleurus much nearer in general character 

 and habit than does A. pustulosa. The same remark also applies 

 to A. spatuligera and A. JDufresnii, the ambulacral structures of 

 which conform so closely to what has been above described, 

 that reference in detail is unnecessary for the present purpose. 

 It is probable that A. alternans, Troschel, will stand in the same 

 category, if indeed the species is really an independent one. 



X. Amlulacra of Arbacia nigra. 

 In concluding these notes on the ambulacra of Arlacia, we 

 would remark that the form known as A. nigra presents an 

 altogether different structure from that of the group of species 

 we have been discussing. The compound ambulacral plates in 

 this species are composed of four or five poriferous plates, and it 

 will be seen on i-eference to fig. 6 that it is the lowest or adoral 

 poriferous plate which is the main primary and occupies the 

 greater portion of the compound plate, the other poriferous 

 plates being all small or demi plates, the main primary plate 

 mounting up and occupying nearly a third of the aboral margin 

 of the compound ambulacral plate. This arrangement, whilst 

 recalling that of some Triplechinidse, has at the same time a 

 definite character of its own. This difierence of structure in our 

 opinion removes A. nigra from the genus Arhacia — a question to 

 which we shall refer in Part II. of this paper. 



XI. Structure of the Vertical Sutures of the Interradia. 

 A noteworthy' system of dovetailing or " dowelliug " occurs in 

 the median interradial suture of the coronal plates in Arhacia. 

 This consists of a series of knobs or eminences situated on the 

 adoral facet of the inner end of the interradial plates, and these 

 fit into corresponding sockets or pits on the aboral facet of the 

 adjacent plates in the neighbouring column. Each coronal plate 

 has consequently knobs on its adoral facet and pits on its aboral 



