66 PROCEEDINGS OF THE NATIONAL MUSEUM. vol.41. 



the origin of certain structures to be observed in crinoids where a loss 

 of the stem normally obtains. Because of this fact they will be dis- 

 cussed in connection with Group II. As will hereafter be noted, the 

 chief interest of these aberrant types is in connection with the origin 

 of the centrale, characteristic of Type 2. 



The genera of the first division may be grouped together according 

 to the way in which the plates of the dorsal cup and the proximal 

 columnals become modified as a result of the loss of their articulated 

 stalks. 



Type 1. — In this type of structure the proximale (and occasionally 

 a few of the columnals) fuses with the infrabasals forming what is 

 known as a centro-dorsal. This centro-dorsal is usually cirriferous 

 throughout the free-swimming stage of the individual. In some forms 

 however, the cirri are lost in the adult. The centro-dorsal also 

 lodges the chambered organ. Some of the genera representing this 

 type of structure are: 



Antedon. 



Actinometra. 



Eudiocrinus. 



Atelecrinus. 



TTiaumatocrinus. 

 Type 2. — Here we find that after the loss of the stem, which is 

 entirely detached from the crown, there is introduced an entirely 

 new element into the dorsal cup. To this plate we shall apply 

 Bather's term centrale. At times, as will be explained later, the 

 centrale may fuse with all or a part of the plates in the proximal 

 circlet of the dorsal cup. This centrale functioned as a plug to stop 

 up the apical opening tlirough which communication was established 

 between the visceral cavity and the stem lumen in the stalked con- 

 dition. The genera illustrating this type of structure are: 

 Vintacrinus. 



Marsupites. 



Saccocoma. 

 Type 3. — In this case again, the stem is entirely lost. In the 

 typical crinoid of this group, Agassizocrinus, the infrabasals, which 

 are quite large and massive, fuse, and together with a secondary 

 deposition of stereom, close the axial canal. Apparently no centrale 

 is formed. Some species of Edriocrinus are structurally analogous 

 to Agassizocrinus as regards the fusion of the proximal circlet, and 

 the secondary deposition of stereom. 



Type 4- — This group has been made for the sake of completeness, 

 to include two Blastoids and certain Cystidea. In these forms no 

 centrale exists nor is there a fusion of the proximal circlet, and a sec- 

 ondary deposition of stereom. Instead, the basals unite very closely, 



