Bd. VI: 4) THE ECHINOIDEA. 77 



been separately acquired by 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. Loven'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 »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 the 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 may 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 30S of A. Agassiz' Fig. 16. Apical area of Abaiits cavernostis, 1,9 mm. 

 »Panamic Deep Sea Echini», representing the apical Simplified copy of PI. XVII. Fig. 9. The genitals 

 area of Abatns cavernosas, (ccrda(us) 1.9 mm. are numbered 1. 2. 3 etc.. the oculars I. II. Ill etc. 



A corresponding stage of the development of Abatus cordatus has been figured 

 by LOVEN (On Pourtalesia. PI. XIV) and by A. AGASSIZ (Panamic Deep Sea Echini 

 PI. 99. Figs. 1 — 7, p. 213, figures 307 — 308) (under the name of Abatus cavernosas). 

 While the explanation of the apical plates given by Lov£n (Op. cit. PI. 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 easily 

 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 PI. XVII. 



