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



based essentially on the presence or absence of a canaliculated structure in the calyx and 

 arm plates." The last of his three leading divisions, the Canaliculata, corresponds to the 

 Articulata, Miiller ; but Chapman's name does not appear to me to be so greatly superior 

 to Mtiller's that it could be adopted without hesitation. Platycrinus, Marsupites, and 

 Uintacrinus have perforated first radials. They are, however, placed l^y Chapman 

 among the Emedullata, in which "the calyx plates are without internal canals." 



The absence of canals in the calyx plates is a character of very general occurrence 

 among the Palseocrinoids, and one which does not present itself in any adult Neocrinoid. 

 But there are several Palseocrinoids in which it does not occur at all, and it cannot there- 

 fore be used as the principal " differentia " for separating the older (Tessellate) from the 

 younger (Articulate) Crinoids. 



Eeasons have already been given for preferring the name " Neocrinoidea " for the 

 latter group to either Stomatocrinoidea or Articulata. The second of these names is 

 certainly founded upon a misconception, and I believe the same to be the case with the 

 first one ; while neither of them was ever properly defined by its author. The name 

 Neocrinoidea is coming gradually into use ; and as the essential differences between this 

 order and that of the Paleeocrinoidea have already been discussed in Chapter X., it is not 

 necessary for me to go into them again, though they may be conveniently put in the 

 form of a definition. 



Order Neocrinoidea. 



Crinoids with a regularly pentamerous calyx, which is generally without primary 

 interradial plates, and except in one genus has no anal or azygous side. The calyx- 

 radials are perforated, and are generally united to the succeeding plates by a muscular 

 articulation. The rays may remain simple, or divide from one to eight times, the first 

 axillary being usually the second joint after the primary radials of the calyx. Orocentral 

 plate probably never developed ; orals, when present, may be limited to larval existence, 

 or remain through life partially covering the peristome, luit capable of being separated 

 so as to open the mouth to the exterior. The oral surface of the visceral mass, 

 with the ambulacra traversing it, may be more or less paved by plates, but is not in 

 any way covered up and shut off from the exterior by a vault. 



