256 Mr. E. Billings on the Structure of 



base of tliis oru;iui must be distant from tlic radial centre at 

 least onc-fom'th of the whole width of the vault. G. aijiho- 

 natus (Hall), figured on the same plate, shows that the anterior 

 grooves curve round to the posterior side of the proboscis, as 

 they do in B. ico^ulacti/lus above cited. 



1 should also state here that, two or three years ago, Mr. 

 Meek, to whom I had Avrittcn forinformaticMi on tliis subject, 

 wrote me that in all cases where he had observed the grooves 

 on the interior of the vault, they radiated, not from the mouth, 

 but from a point " in front of it." (This would not be in front 

 of, but behind, the mouth, according to the terminology used 

 in these notes. I tlnnk that the side in which the mouth is 

 situated should be called " anterior" or "oral," even although 

 both the mouth and anus should be included in it.) 



In all the species above cited, the figures (with the exception 

 of that of C. ornatus) exhibit the relative position of the mouth 

 and radial centi'e as it has been actually seen in casts of the inte- 

 rior of the vault. But, besides these, numerous examples may 

 be found in the works of Miller, Austin, De Koninck, Phillips, 

 Meek, Worthen, Shumard, Hall, Lyon, Cassiday, and others, 

 of Crinoids whose external characters show that, in them, the 

 mouth cannot be in the central point from which the grooves 

 radiate. 



With respect to Prof. Thomson's theoiy, I freely admit 

 that, if it is true that in all the Echinodermata, fossil and 

 recent, the mouth is the radial centre, then that aperture must 

 be the one which I call the ambulacral orifice in the Cystidea. 

 The views, however, advocated by me in my Decade No. 3 

 appear to be gradually gaining gromid. As these fossils are 

 rare, few have occasion to study them ; and consequently the 

 subject has not been much discussed since 1858, the date of 

 the publication of that work. The following are the only 

 authors, so far as I have ascertained, Avho have given their 

 opinions on this vexed question during the last eleven years : — 



Prof. Wyville Thomson, op. cit. p. Ill (1861), agrees with 

 me that the lateral aperture is not an ovarian orifice, but, as 

 we have seen, is strongly opposed to the view that it is the 

 mouth. He calls it the anus. 



Prof. Dana (Man. Geol. p. 162, 1863) recognizes it as the 

 homologue of the simjde aperture (oral and anal) in the sum- 

 mit of tho.se Crinoids which have but one. Tliis is exactly my 

 view. [J.W. Salter agrees with Prof. Thomson that it is the 

 anus, not the ovarian apei-tm-e (Mem, Geol. 8ur. G. B. vol. iii. 

 p. 286, 1866.) Prof. S. Lovdn, of Stockholm, has described, 

 in the ' Proceedings of the Royal Swedish Academy,' 1867, the 

 remarkable sea-urchin, Zei.^ /a rnirahiUs (Gray), which has the 



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