ECHINODERMATA : CRINOIDEA. 



211 



posed of a number of calcareous plates, accurately fitted to- 

 gether, and having the following general arrangement (fig. 104). 

 Resting directly upon the summit of the highest joint of the 



Fig. 103 Platycrinus tricontadactylus. Carboniferous. The left-hand figure shows 

 the calyx, arms, and upper part of the stem ; and the figure next this shows the sur- 

 face of one of the joints of the column. The right-hand figure shows the proboscis. 



column is a series of plates, generally three or five in number, 

 which from their position are termed the " basals " (fig. 104, b\ 

 Succeeding to the basals, and alternating with them, there is 

 commonly found a second cycle of polygonal plates, which are 

 generally termed the " parabasals " (fig. 104,^), and which in 

 many forms are never developed.* Succeeding to the para- 

 basals (or, in the absence of these, to the basals) are two or 

 three cycles of plates, which are directly superimposed upon one 

 another in longitudinal rows, and which form the foundations 

 of the arms. These are known as the "radials" (fig. 104, r\ 

 and are termed "primary radials," "secondary radials," and 

 "tertiary radials," according to their distance from the basals. 

 The last radial plates, or those furthest from the column, give 



* According to the high authority of Mr P. H. Carpenter, when there is 

 only one cycle of plates between the top column-joint and the primary 

 radials, it is the so-called "basals" (or "under-basals") which are want- 

 ing, and the cycle that is present consists of plates corresponding with the 

 " parabasals " of such Crinoids as have two inferior cycles of calycine plates. 



