348 PROCEEDINGS OF THE ACADEMY OF [1888, 



of theories, even though some of our own views suffer in consequence. 

 We also take pleasure in bearing this testimony to the soundness of 

 Dr. P. H. Carpenter's views as to the nature of the ventral covering 

 in the Ichthyocrinidae. He always considered that this family rep- 

 resented an approximation to the Neocrinoids, and that the integu- 

 ment was comparable to a disk and not to a vault.' 



This discovery is also a confirmation of the opinion always insist- 

 ed upon by us, as a conclusion necessarily following from the struct- 

 ure of the calyx and arms, that the ventral covering of the Ichthyo- 

 crinidae was pliable, yielding to motion in the calyx and arms, and 

 emphasizes the distinction between this group and other Palaeozoic 

 Crinoids based on the summit structure, as pointed out by us at the 

 beginning of our writings (Rev. I, p. 5), although, we admit, to a 

 higher degree than we ever anticipated. 



Recurring now to the orals, it is easy enough to understand from 

 the structure of Taxocrinus how a set of five equal plates, symmet- 

 rically disposed over the mouth as in the larva of Antedon, could be 

 so altered by the presence of anal structures, as to bring the mouth 

 beneath the i^osterior plate. It is readily conceivable, that by the 

 encroachment of the anal plate, the posterior oral was pushed to a 

 central position, and remained permanently in that condition. The 

 transition from five unequal to five equal orals through such forms a& 

 Platycrinus (PI. XVIII, fig. 15), seems also quite apparent. The 

 fact that the covering plates of the ambulacra in our specimen rest 

 against the lateral edges of the orals, is contrary to the observations 

 heretofore made among recent crinoids in which orals have been 

 observed. In all of them the ambulacra pass in at their outer mar- 

 gins, and the plates are parted so as to form open slits. In the 

 Camarata the orals remain closed, and the ambulacra, — when ex- 

 posed at all, — with their food grooves closed, enter the vault on or 

 before approaching the orals. 



We therefore consider the evidence entirely conclusive that the- 

 homologues of the five oral plates of the young Antedon and tlie 

 adult Holopus, Hyocrimis, Rhizocrinus and ThaumatoGnnus are to be 

 found in the so-called central plate and four large proximals in all 

 Camarata in which these are developed — the two smaller proximals, 

 heretofore considered as the equivalent of a fifth, being anal plates. 



The question now naturally arises, what are the morphological 



1 Challenger Report on the Stalked Crinoids, pp. 42, 181 and 182, and else- 

 where. 



