24f> PROCEEDINGS OF TUE ACADEMY OF [1885 



that the vault of a Platycrinite was exactly of the same nature 

 as thai oi an A.ctinocrinite, i. e., that it covered in the whole of 

 the visceral inns- and ambulacra on its upper surface. For if the 

 alternating dome plates represent the covering plates of recent 

 Crinoids, then all the periphery of the dome, outside of the apical 

 dome plates, must be the real ventral surface of the body, and 

 not a tegmen calicis as in Actinocrinus." And he states further, 

 on page 1 T ' > : " There is some point on the actinal side of every 

 Crinoid where the food grooves leave the oral system, covering 

 up the peristome in which the}- originate, and are only closed by 

 the covering plates a1 their sides." This is quite true as to the 

 Neocrinoidea, in which the calyx- is limited to the dorsal side, but 

 not altogether in the case of the older Crinoids, in which the calyx. 

 as we believe, takes up the greater part of the ventral surface, 

 and the covering pieces frequently are embodied among abactinal 

 plates. In the Platycrinidse the disk is subtegminal, although 

 portions of the covering pieces appear along the surface, but 

 these, in place of lining the sides of the food grooves, are incor- 

 porated between the interradials, resting between them as solidly 

 as the summit plates, and cover the food grooves as tightly, as 

 the interradials do in Actinocrin"*. 



Carpenter agrees with us that the radials above the first are 

 fundamentally arm plates, which, in the growing Crinoid, by the 

 increase of interradials, were incorporated into the calyx. 

 During the process of incorporation, by the widening of the 

 equatorial zone, the ambulacral vessels and food grooves of the 

 incorporated arm plates, gradually were lifted .out from the arm- 

 furrows, and stretched out, along the disk in the form of tubes, 

 being enclosed from above and below by plates. These ambula- 

 cral tubes in most of the Actinocrinidse are altogether subteg- 

 minal, and located at a distance from the inner floor of the vault, 

 until on approaching the arm bases they not only come in contact 

 with, but raise up the interradial plates and push them aside, 

 exposing to view the upper rows of tube plates, the so-called 

 covering pieces, which are thence continued along the arm 

 furrows. 



In the Platycrinidse, the conditions are essentially the same as 

 in the Actinocrinidfe, hut most generally the covering-plates of 

 the tubes penetrate the vault before they pass into the arms. 

 This takes place either along the outer edges of the proximals, 



