172 THE VOYAGE OF H.M.S. CHALLENGEE. 



while in Periechocrinus the vault is composed " of small irregularly arranged smooth 

 pieces, among which the apical dome plates are indistinctly represented."^ 



All these genera, therefore, resemble Strotocrinus, and from the evidence of that genus 

 we are entitled to believe that the apical dome plates would be relatively more prominent 

 at earlier stages of development. Now this is exactly the case with the orals of recent 

 Crinoids, which at first surround the whole of the upper surface of the larval body ; but 

 eventually may be altogether resorbed or gradually reduced in comparison with their 

 fellows, the basals, as in Rhizocrinus (PI. X. figs. 2, 3, 7). 



Thus then I believe the oral or actinal system forming the vault of Actinocrinus to 

 have been developed on the left larval antimer, in exactly the same way as the apical or 

 abactinal system is developed on the right ; but the oral system, instead of being limited 

 to five oral plates as in Neocrinoids, reached a very extensive development, so that in its 

 completest form it presents such a parallel to the apical or abactinal system as is to be 

 met with in no other Crinoid, much less in any other Echinoderm. 



The greater variability in the development of the proximal plates, and theii- 

 occasional separation by smaller intercalated pieces, resembles the extreme irregularity 

 of the apical system of an Ophiurid, as compared with that of a Crinoid or Urchin.^ The 

 interradial plates of the former (or basals) have important relations to the chambered 

 organ. Those of the Urchin (genitals) are often connected with the genital ducts ; and 

 the radial plates in the same way have important functions in both groups. In the disk 

 of an Ophiurid, however, neither interradial nor radial plates are of any functional 

 importance ; and we find accordingly that their state of development difi'ers very 

 considerably even in allied species. Much the same is the case in the dome of the 

 ActinocrinidaB, where the plates are not in any way specially related to internal organs, 

 though serving to protect them. 



Wachsmuth totally denies that there is any homology between the solid vault of a 

 Palseocrinoid and the ventral or oral disk of a recent form. So far as the Actinocrinidse 

 are concerned, I entirely concur in this opinion, except as regards those Neocrinoids, 

 such as Hyocrinus, Thaumatocrinus, and Holopus, which have persistent orals. For I 

 believe that representatives of these plates exist in the vault of all PalEeocrinoids, 

 whether simple or complex, although they are sometimes very greatly reduced ; and I 

 cannot therefore agree Ydth. ZitteP in considering them as entirely absent in the vault of 

 Actinocrinidse, Platycrinida^, Melocrinida3, Rhodocrinid^, &c. 



The Cyathocrinidge and the Blastoids seem to resemble Strotocrinus and Periecho- 

 crmus in the small size and want of definite arrangement of the apical dome plates. 

 Neither in Wachsmuth's famous specimen of Cyathocrinus malvaceus, nor in any of the 

 Cyathocrinidae figured in Angelin's Iconographia can any definite arrangement of the 



1 Revision, part ii. p. 131. ^ Quart. Journ. Micr. Sci., 18S4, vol. xxiv., N. S., pp. 4-14. 



^ Palseontologie, p. 331. 



