194 PROCEEDINGS OF THE ACADEMY OF * [1881. 



sufficient to mark the two structures, but this should be done 

 uniformly and we shall accordingly propose subgenera where 

 necessary for this purpose. 



The vault does not completely cover the calyx, but leaves along 

 the line of junction a row of oval or circular passages which have 

 been called arms or " ambulacral openings." The belt in which 

 they occur is known as the " arm regions," and their distribution 

 in the different rays is expressed by the " arm formula." Thi'ough 

 the arm openings which are veiy conspicuous in the Sphaeroido- 

 cnnidii:', food entered the body, and the}^ served as passages for 

 the ambulacral vessels. In a mature specimen the number of 

 primar}^ arms can be ascertained by counting the arm openings ; 

 but not always in young specimens, or in species in which the 

 radial portions are extended into free ra^'s. 



By " free rays " we mean lateral extensions of the body, com- 

 posed of a succession of radials, unconnected by interradials, and 

 covered with similar plates, as solidly and in the same manner as 

 the radial portions of the dome proper. These free rays, whether 

 composed of only a few plates as in Plafycrinus, or extended 

 almost to the full length of the arms as in Eucladocrinus and 

 Steganocrinus, are actually portions of the body, and the arms are 

 given off from them in the same manner as from the body in other 

 cases. 



In the PlatyCrinid^ generally , there are within the calyx primary 

 radials onlj^, all the higher orders of radials being included within 

 the free ra3^s. In the body of a Platycrinus in its ordinary preser- 

 vation, we find but five arm openings, but whenever the bifurcating 

 plate is well preserved at its distal end, ten openings are Ausible,^ 

 and these form passages for the free rays which here divide, each 

 branch giving off arms laterally and from opposite sides. The 

 free rays are rarely preserved in the fossil unless the arms are 

 attached, when they really appear like arms and have been 

 described as such. That they are not arms is proved by the fact 

 that their ventral side is not provided with a furrow, but is 

 covered in the same way as the vault proper. In these forms, as 

 might be expected, the number of arms cannot be determined from 



^ This proves that P. Herbert Carpenter is correct in saying that the 

 division of the arms actually begins at the middle of the bifurcating plates, 

 (pn Actinometra, p. 22). 



