140 ECHINODERMA. CRINOIDEA 



the earlier rows of brachial plates become firmly united 

 to one another and to the radials (figs. 56, 58, 2, 3, br) ; 

 these fixed brachials have often been regarded as radials, 

 but morphologically they are only brachials which have 

 become incorporated into the calyx. The fixed brachials 

 may be in contact at the sides, or, as in most Palaeozoic 

 crinoids, they may be separated by other plates which are 

 termed inter -brachials (fig. 56, ir). In the posterior inter- 

 radial area (that which leads up to the anus) the inter- 

 brachial plates are often more numerous than in the other 

 areas. When a stem is absent there is often a central 

 plate at the base of the calyx {e.g. Marsupites). 



The tegmen or oral surface of the calyx is usually more 

 or less completely covered by calcareous plates. Sometimes 

 five large triangular plates (orals) only occur, between 

 which are the food-grooves ; but usually other smaller 

 plates are also present — the food-grooves being usually 

 covered by plates, sometimes called " ambulacrals," and 

 between them occur numerous " interambulacral" plates. 

 In many Palaeozoic crinoids (e.g. Actinocrinus) the tegmen 

 consists of a complete vault or dome of stout plates 

 concealing the mouth as well as the food-grooves and 

 their covering plates; commonly this plated tegmen 

 extends upwards around the anal process forming a tube- 

 like covering. 



In the genera described below, the basals, radials, and arms are 

 five in number unless otherwise stated. 



A. Monocyclic Crinoids 



Platycrinus. Basals three, unequal. Radials large. Some 

 fixed brachials. One inter-brachial in each area — more in the 

 posterior (anal) area. No inter-radial. Arms bifurcating once to 

 thrice, uniserial at the lower end, biserial above ; pinnules long. 



