Bd. VI: 4) 



THE ECHINOIDEA. 



n 



beeil separately acquired hy the Salenids and Echinids proper, which leads to the 

 conclusion that the alleged homology between this plate and the dorsocentral plate 

 of the Crinoids is false. LovÉn's suggestion, that the small plates on the periproct 

 are »the rudiments of the central ossicle and the costal 5» (On Pourtalesia p. 74. 

 PI. XIV. Fig. 164), seems unacceptable, resting, in fact, on no evidence at all, only 

 on the a priori assumption that the central plate must be represented in some way. 

 (Regarding the j'Costal» 5 see below). Still less acceptable appears his explanation 

 of the madreporite of the Ethmolytic Spatangoids as being composed of genital 

 (costal) 2, the central plate and genital (costal) 5 (On Pourtalesia p. 71). The exi- 

 stence of a separate genital 5 proves that the madreporite has not absorbed this 

 plate, as it were; it has, upon tlie whole, nothing to do with this plate. Unfor- 

 tunately, I am unable to state beyond doubt the ultimate fate of genital 5. There 

 are only two alternatives, viz. that it may be resorbed and disappear totally, or that 

 it ma)' remain as the posterior of the periproctal plates. I think the latter alter- 

 native is the more probable, but I cannot give definite proof for it. 



Fig. 15. Copy of the figure 308 of A. AgaSSIz' 



»Panamic Deep Sea Echini», representing the apical 



area of Abaliis cavernosus, {coriiatus) 1,9 mm. 



Fig. 16. Apical area of Abattis cavernosus. 1,9 mm. 

 Simplified copy of PI. XVII. Fig. 9. The genitals 

 are numbered I, 2. 3 etc., the oculars J, II, III etc. 



A corresponding stage of the development of Abatiis coniatus has been figured 

 b}' LovÉN (On Pourtalesia. PI. XIV) and by A. Agassiz (Panamic Deep Sea Echini 

 PI. 99. F"igs. I — 7, p. 213, figures 307 — 308) (under the name o{ Abatns cavernosus). 

 While the explanation of the apical plates given by LovÉN (Op. cit. PL XIV. Fig. 

 164 a) is evidently correct, as far as he has been able to trace the plates, the ex- 

 planation of these plates given by Agassiz is evidently unacceptable, as is easilj- 

 seen on comparing the figures 15 and 16, the former representing the figure 308 

 from the Panamic Deep Sea Echini», the latter a simplified copy of the PL XVII. 



