348 PROCEEDINGS OF THE ACADEMY OF [1890. 



Physetocrinus, which is but a modified Actinocrinus, in some species 

 has orals and radial dome plates, while in others the whole ventral 

 surface is covered by numerous irregular pieces. Actinocrinus 

 multiradiatus, on the contrary, has but few very large ventral plates 

 which interlock with those between the rays. In most of the Actino- 

 crinidae the interradials pass insensibly into the vault, there being 

 no dividing line ; while in Batocrinus generally, but not always, the 

 interradials of the dorsal side are distinctly separated from those of 

 the ventral side by the overarching brachials, a structure w'hich led 

 us at first to suppose the plates of the two sides to be morpho- 

 logically distinct. 



Similar diflferentiations we find in the ventral structure of the 

 Platycrinidae and Hexacrinidae. In some of their species the pave- 

 ment is made up entirely of massive plates, in others of compara- 

 tively thin pieces ; while in still others the ventral surface is occupied 

 almost exclusively by the orals. In both these groups absolutely 

 no distinction can be made between interradials and vault plates. 

 The first row, which generally consists of three plates, is peripheral, 

 and is followed by other rows which are strictly ventral. The plates 

 forming the second and upper rows, when such are present, inter- 

 lock with each other and those of the first row, in a similar manner 

 as the interradial plates of the dorsal cup in an Actinocrinus. 



The conditions of the ventral pavement in the Melocrinidae, Rhodo- 

 crinidae and Glyptasteridae are very similar to those in the Actino- 

 crinidae and Platycrinidae ; many of them have uninterrupted rows 

 of covering pieces exposed upon the surface, but the plates as a rule 

 are smaller, less regular in their arrangement, and the orals and 

 radial dome plates are more rarely represented. The lower inter- 

 radials in all of them are definitely arranged, and there is no line of 

 demarkation between the two hemispheres except that produced by 

 the arms which pass out between them. In the Reteocrinidae, as 

 in most of the Silurian Camerata, the whole ventral surface is 

 covered by minute irregular pieces, and similar plates, with a few 

 somewhat larger ones scattered among them, are interposed between 

 the rays from the basals up. In the Grotalocrinidae and Acro- 

 crinidae, the calyx ambulacra are exposed ; their covering pieces are 

 comparatively small, and remarkably regular in their arrangement. 



Dr. P. H. Carpenter (Chall. Rep. on Stalk. Crin., pp. 165 and 

 166) agrees with us that the calcareous network beneath the vault 

 of an Actinocrinus " corresponds to the limestone j^articles on the 



