108 THE CRINOIDEA CAMERATA OF NORTH AMERICA. 



Carpenter ^ held the same opinion as to the vault of the ActinocrinidaB, 

 but he believed that the " vault " of a Platycrinoid ^' corresponds collectively 

 to the orals, interradials, ambulacral and anambulacral plates of Neocri- 

 noids." He agreed with usf that the calcareous network beneath the 

 " vault " of an Adinocrinus " corresponds to the limestone particles on the 

 surface of the internal casts, and represents the anambulacral plates de- 

 veloped in the perisome of recent Crinoids." He also admitted " the com- 

 plete resemblance between the ventral perisome of a recent Crinoid and the 

 upper surface of the body beneath the vault of an Adinocrinus.'" '' The vault 

 and ventral disk/' he said, ''are to my mind entirely distinct structures." 

 Of the vault, he said further (p. 172) : "I believe the oral or actinal system 

 forming the vault of Adinocrinus to have been developed on the left larval 

 antemer, 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 ^nq 

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

 in its completest form it represents such a parallel to the apical or abactinal 

 system as is to be met with in no other Crinoid." From these passages and 

 others in the Challenger Eeport, it appears that Carpenter, as well as we, 

 supposed that in Adinocrinus all plates of the calyx up to the arm bases were 

 abactinal, and all constituting the ventral side actinal ; not only the orals 

 and radial dome plates, but also the so-called interradial dome plates or 

 interambulacral pieces. 



We retained the above mentioned opinion until] 881; but afterwards 

 our views were materially changed respecting the interradial dome plates,:}: 

 which we no longer regarded as actinal structures, but took to be con- 

 tinuations of the interradial plates of the dorsal cup, and as fundamentally 

 identical with them. In considering the matter in the first place, we had 

 not overlooked the fact that in many of these Crinoids, throughout different 

 groups, the covering plates of the ambulacra are exposed at the surface, and 

 as such would form a part of the '-' vault." 



It is a striking fact, in the Crinoids as elsewhere, that some characteristics 

 which are of the greatest importance from a morphological point of view, 

 prove to be of comparatively little value for classification. This is the case 

 in a marked degree with the ambulacra of the Platycrinidge and Actino- 

 crinidae, which are found to be tegminal and also subtegminal. In the Platy- 



* Chall. Rep. Stalked Crinoids, pp. 172-180. 

 t Chall. Rep. Stalked Crinoids, pp. 165, 166. 

 % Revision, Part III. pp. 16-27 (Acad. Nat. Sci. Phila., pp. 238-249). 



