\ 



168 THE VOYAGE OF H.M.S. CHALLENGER. 



side, which are of equally fuudameutal importance in the calyx.^ " The six proximal 

 plates surrounding the central piece represent the basals or genitals, and the radial dome 

 plates the radials or oculars. The centre piece may perhaps be compared with the 

 underbasals, or the subanal jalate of the Echini." In an earlier statement of these 

 homologies^ no "perhaps" was used respecting the nature of the central actinal plate ; 

 and I have pointed out that while accepting Wachsmuth's^ comparison to the full extent, 

 as regards the radial and interradial plates in the centre of the dome, I cannot follow him 

 in his recognition of the dorsocentral, nor of its fellow, the orocentral, as homologous with 

 under-basals. * His views, as expressed in the Eevision, are essentially those of Lovfen, 

 formerly also held by Agassiz, when allowance is made for the different systems of 

 nomenclature used by him and by them. I have endeavoured to show elsewhere that 

 there cannot be a true homology between a dorsocentral plate which is single from the 

 first, and the five under-basals of a dicyclic Crinoid. These are by no means universally 

 j)resent, as one would expect them to be, did they correspond to the dorsocentral of the 

 Eehinozoa, which is such a prominent object in the larva of an Urchin, Starfish, or 

 Ophiurid ; while representatives of the under-basals of Crinoids are actually present, to- 

 gether with a dorsocentral plate, in some Starfishes and Ophiurids.* The dorsocentral 

 is developed at the distal end of the right peritoneal tube ; and as there is a plate 

 occupying the same position in the Crinoid larva, viz., the future terminal plate at the 

 base of the stem, it is only natural to regard the two as homologous, as pointed out by 

 myself in 1878. I am glad to find that this view has commended itself to Dr. Ltitken' 

 and also to Sladen ; and I understand from Mr. Wachsmuth that he is now in accordance 

 with me respecting the homologies of the central vault piece, considering it as representing 

 the dorsocentral of Echinoderms generally, and not the under-basals of those Crinoids in 

 which these plates occur. [See Appendix, Note A.] His view of the proximal inter- 

 radial dome plates of the Actinocrinidte, however, is entirely difi"erent from that here 

 advocated, and will shortly be published in extenso by himself; while he has also 

 abandoned his suggestion that the interradial dome plates in the Actinocrinidee, Platy- 

 crinidae, and Rhodocrinidse are " the homologues of the oral plates, which are here broken 

 up, and represented by five plates instead of one." This relieves me from the necessity 

 of discussing it here, as I had otherwise intended to do. 



My own idea of the homologies of the calyx and dome plates of Crinoids is expressed 

 in Table VII. 



1 Revision, part ii. pp. 15, 16. ^ Amer. Journ. Sci. and Arts, vol. xiv. \>. 189. 



2 Quart. Journ. Mia: Sci., 1878, N. S., vol. xvui. pp. 369-371 ; 1879, vol. xix. pp. 181, 182 ; 1882, vol. xxii. 

 pp. 377, 378. 



* See Sladen, On the Homologies of the Primary L;u-val Plates in the Test of Brachiate Echinoderms, Quart. 

 Journ. Micr. Sci., 1884, vol. xxiv., N. S., pp. 32-36 ; also. The Apical System of Ophiurids, ibid., pp. 3-15. 

 ' Dyreriget, en Haand- og Lferebog til Brug ved h0iere Lajreanstalter, Kjpbenhavn, 1882, p. 597. 

 " Eevision, part ii. p. 17. 



