266 PROCEEDINGS OP THE ACADEMY OP [1885. 



are strong, rigid, without pores, are suturally connected, and their 

 arrangement is irregular. This tube differs essentially from the 

 ventral sac, which forms a part of the disk, and is composed of 

 anambulacral plates, into which the plates of the abactinal tube 

 arc incorporated, in a similar manner as the higher radials and 

 proximal pinnules are into the disk of the Neocrinoidea. More- 

 over, the ventral sac does not contain the anal aperture, which is 

 general^ located within the disk. For further consideration of 

 this organ we refer to our chapter on the perisomic plates. 



Among the Articulata, the Crotalocrinidae and Cleiocrinidoe 

 have an anal plate in lateral contact with the radials. The same 

 is the case in the Ichthyocrinidse, with the exception of Ichthyo- 

 crinas, which has dorsally no anal plate and generally no inter- 

 radials. Pycnosaccus, Calpiocrinus, Homalocrinus, Lecano- 

 crinus, Gnorimocrinus and Mespilocrinus have even an azygous 

 piece, which is absent in Anisocrinus, Taxocrinus, Onycho- 

 crinus, Forbesiocrinus and Lithocrinus. An anal appendage has 

 been observed only in Crotalocrinus and Enallocrinus, located 

 ventrally, close to the periphery. In the former it consists of a 

 tube composed of eight vertical rows of heavy quadrangular 

 pieces, connected by suture. In Enallocrinus its form is 

 unknown. 



In Part I we described Taxocrinus, Onychocrinns and Gnori- 

 mocrinus as having a small lateral tube resting upon the first 

 anal plate. To this Dr. P. H. Carpenter objected in his paper 

 on Thaumatocrinus (Philos. Trans. Royal Soc, 1884, pt. iii, p. 

 928). He admitted " that the arm-like series supported the lower 

 portion of the anal interradius," but doubted " that the plates 

 had been hollowed out on their inner side for the reception of 

 the hind-gut," which " undoubtedly opened to the exterior at a 

 higher level through a regular anal tube, just as in other Crinoids." 

 These objections are well founded, and we are now fully con- 

 vinced that those plates were bordered laterally by interradial 

 pieces as in Reteocrinus. 



The Plates of the Actinal System. 



A. The Summit Plates. 



The summit plates consist of the actinal plates, overlying and 



immediately surrounding the peristome. For these plates we 



have heretofore proposed the name " apical dome plates," but 



