NOTES ON ECHINODERM MORPHOLOGY. 377 



radial in position ; then the interradial basals, and then the 

 radials. Outside and alternating with these must have come 

 the orals, one of them originally perforated by the primary 

 water-pore. 



The homology of the dorsocentral plate with the subanal 

 plate of the Urchins was, I believe, first noticed by Loven,^ and 

 so far as I know no one, with the important exception of 

 Ludwig, has denied it. 



There have, however, been differences of opinion respecting 

 the homology of this dorsocentral plate in the other Crinoids, 

 Agassiz," Loven, and more recently also Wachsmuth/ regard it 

 as representing the centrodorsal of the monocyclic Comatida 

 and the under-basals of the pedunculate Crinoids which have a 

 dicyclic base. They do not, however, give it a homologue in 

 Fentacrinus, Flaiycrinibs, and the numerous other Crinoids in 

 which the basals (= genitals) rest directly upon the top stem- 

 joint, and there are no under-basals. Wachsmuth gives an 

 instance of the metamorphosis of the five basals of Edriocrimis 

 into one plate during the life of the individual as " a calcareous 

 deposit is secreted around the base, which covers and obliterates 

 the sutures between the plates." Much the same thing happens 

 with the under-basals of Agassizocrinus, so that in most of the 

 adult specimens " not even a vestige of the sutures formerly 

 existing between the plates can be detected," and this " actual 

 metamorphosis — during the life of the individual — of five plates 

 into one seems to us to be strongly confirmatory of the views of 

 Agassiz and Loven." 



I cannot admit, however, that the obliteration of the sutures 

 between the five under-basals of Agassi zocrinus is any proof of 

 the collective homology of these plates with the primitively 

 single dorsocentral of an Urchin ; and Wachsmuth's attempt to 

 support this position by instancing the fusion of the basals in 

 Edriocrinus is hardly satisfactory. A similar obliteration of the 

 inter-basal sutures occurs in Holopus, Rhizocrinns, and Bathy- 

 crinus, but Wachsmuth would hardly be prepared to say that 

 the basals of these Crinoids are therefore homologous with the 

 dorsocentral of an Urchin ; and the logical result of his argu- 

 ment would be that the basals of Edriocrimts are homo- 

 logous with the under-basals of Agassizocrinus, which I am con- 

 fident he would never think of asserting. 



For my own part, I have already pointed out^ that there 



' " Etudes sur les Echinoidees," ' Kongl. Svenska Vetenskaps Akademiens 

 Handlingar,' Bd. ii, No. 7, p. 65, sqq. 



' ' Embryology of the Starash,' pp. 62, 63. 



^ 'Revision of the Palseocrinoidea,' part i, Philadelphia, 1S79, p. 21. 



•• This Journal, vol. xviii, 1878, pp. 371, 372. 



