CBINOIDEA. 



277 



ceeded by a cycle of three parabasals. There are five radials, 

 two of which stand directly upon the basals, while the others 

 rest on the parabasals. The upper surface is covered by 

 five inter-radials, so disposed as to form a pyramid. In the 

 related genus Trwcrinus, of the Silurian and Devonian, there 

 are only three basals, and a single parabasal. 



We come now to a series of the stalked Crinoids in which 

 there are no parabasal plates. Foremost among these is the 

 great family of the Actinocrinidce, of which Actinocrinus itself 

 is the type. In this genus the calyx (figs. 153-155), though 

 very variable in shape, consists of three basals (fig. 165, b), 

 which articulate upwards with three cycles of radials, the 

 " sub-radials " being wanting. The axillary radials carry each 

 a double series of brachial plates, which support the vari- 

 ously divided arms. There are three or more' anal plates, 

 of which the lowest (fig. 165, a) always rests upon the 

 basals directly. There is a variable number of inter-radials, 

 and the column is round. The upper surface of the cup is 

 vaulted over with calcare- 

 ous plates, and the brachial 

 grooves are continued be- 

 neath the vault thus formed, 

 as so many tunnels, to the 

 central and concealed mouth. 

 The anus may or may not be 

 extended into a proboscis, 

 and it is sometimes very 

 excentric, sometimes sub- 

 central. It has been shown 

 that in many of the Actin- 

 ocrinidce there exists in the 



interior Of the Calyx a Sin- Fig . i65.-Diagram of the dissected calyx of 

 gular Convoluted CalcareOUS ^<f (after Schultze) & Basals; r, 



Radials ; i, Inter-radials ; a, The lowest of the 



plate, of a reticulated tex- anai plates. 



ture, shaped somewhat like 



an ordinary Bubble-shell (Bulla), occupying the vertical axis 



of the body, and often of large size. This has been compared 



with the calcareous structures present in the " sand-canal " 



of various Echinoderms ; but it is probably rather an extreme 



