360 I'ROCEEDINGy OF TIIK ACADEMY OF [1890. 



ing that these plates represent morphologically the perisomic plates 

 which cover the so-called interradials of Cyathocrinus. 



Now, is it not possible that the deltoids are compound structures, 

 and the concealed parts were originally separate siibamhulacral 

 plates, which in palaeontological times became anchylosed with the 

 deltoid pai't to give more strength to the calyx ? This seems quite 

 probable if we consider that a fusion of two or more plates often 

 takes place among Crinoids, and that interradial plates, as the term 

 denotes, cannot be suhambulacral. Such, indeed, also might have 

 been the case in Easpirocrinus in which the interambulacral parts 

 are large and apparently not covered by perisome. It is in this re- 

 gard very significant that in Blastoids with large deltoids, such as 

 Elaeacrinus and Schizoblastus sayi, it appears from the ornamenta- 

 tion as if the deltoids originally might have consisted of three parts 

 which afterwards had been anchylosed, and Lyon (Kentucky Geol. 

 Rep., p. 489, PI. V, fig. 16), has actually described interradials in 

 Elaeacrinus. In this genus we find generally upon the surface of 

 the deltoids, parallel with the ambulacra, a short distance from them, 

 raised lines, and all striations toward them from the ambulacra are 

 horizontal, while those between the lines take an upward course. 

 This feature is so universal and conspicuous, that it probably is more 

 than mere ornamentation. Such striations upon the plates are more 

 or less the result of growth, and this would seem to indicate, that 

 the growth of the plates took place independently in different direc- 

 tions, and that the parts in question are morphologically distinct. 

 The ambulacra of Elaeacrinus are narrow, and it is quite possible 

 that in this genus the " subambulacral " pieces enclosed an interra- 

 dial in the way we suggested ; but that in Cyathocrinus they abutted 

 laterally so as to form a support for the disk. This of course is as 

 yet simply hypothesis and has to be substantiated. 



The ventral sac of the Cyathocrinidae rarely, if ever, reaches the 

 tips of the arms. It is closed at the up})er end, and composed of ver- 

 tical rows of hexagonal, rather short and solid pieces, which are so 

 arranged that the plates of adjoining rows alternate with one an- 

 other. The anus is placed near the top facing anteriorly, and closed 

 by a small pyramid of five or six triangular pieces, resembling those 

 of the Cystids and Stephanocrinus. 



The tegmeu of the Hybocrinidae is similar to that of the Cyatho- 

 crinidae, but the posterior interambulcral area rises but little above 

 the level of the four others. The ambulacra are tegminal, and ap- 



