REPORT ON THE CRINOIDEA. 157 



and its allies as forming "a free arch which braces the entire oral side of the body 

 without the aid of oral plates." 



The use of the term " aboral" as applied to the vault is somewhat unfortunate, for it 

 actually does cover in the mouth ; while the j^lates of the opposite or dorsal side of the 

 body have generally been called aboral by writers on Echinoderm morphology. Wach- 

 smuth himself applies this expression to the plates of the cup up to the level of the arms. 

 In like manner he gives the name " apical dome plates " ^ to " a system of plates in the 

 vault which occupy a position analogous to that of the apical plates of the calyx ; " viz., a 

 central plate, with a proximal ring of interradial, and a distal ring of radial plates disposed 

 regularly around it. These must be carefully distinguished from the dorsocentral, basals, 

 and radials, which are the apical plates of the calyx ; and, as mentioned above, have definite 

 homologues in the apical system of Urchins and Stellerids. [See Appendix, Note A. j 



The suggestion of Wachsmuth and Goette that the Palaeocrinoids represent a 

 comparatively early stage in Crinoid ontogeny, before the opening of the tentacular 

 vestibule to the exterior, has been very generally accepted. But it must be borne in 

 mind that though the Palaeocrinoids may be considered as permanent larval forms with 

 respect to the closure of the actinal side, yet that in other respects they have developed 

 to a far greater extent than any Neocrinoid. The solid vault of an Actinocrinus is a 

 structure sui generis, unless, as I believe, its proximal ring of interradial plates is 

 represented by the orals of a Neocrinoid. The extraordinary development of arms or of 

 other appendages which we find in forms like CaUicrinus, Pterotocrinus, Ollacrinus, 

 Eucalyptocnnus, Crotalocrinus, &c., is entirely without a parallel among the more 

 regular and symmetrical Neocrinoidea. We must be careful therefore not to make too 

 much of the one or two embryonic characters presented by the Pala^ocrinoids, as com- 

 pared mth the facts of their great complexity of structure and immense variety of form. 



The simplest type of summit to be met with in any Pala30crinoid is that presented by 

 the Devonian genus Haplocrinus, which remains permanently in the condition of a very 

 early larva. For the orals, together with certain upward processes of the radials on 

 which they rest, form a closed pyramid just as in an early Pentacrinoid. There are five 

 openings which lead in Ijeneath this oral pyramid and correspond to the points of 

 attachment of the arms ; but its apex is completely closed so that there is no external 

 mouth. The lines of suture between its component plates are generally marked by deep 

 grooves which descend from the closed apex to end below at the radial openings. 

 Wachsmuth and Springer call them compartments for the reception of the arms, while 

 they have been described by Zittel ^ as " nach unten geschlossene, nach oben offene 

 Ambulacralf urchen. " 



This appears to me to be a mistake, and I do not see any reason for supposing that 

 these orals were ever covered in by plates as he elsewhere suggests. 



1 Revision, part i. p. 28. ^ Palaeontologie, p. 347. 



