426 



PELMATOZOA. 



— i.e., they may consist of a single or double row of brachials — but 

 they are most commonly the latter. The ventral surface of the 

 calyx is covered with a vault of heavy plates which are closely con- 

 nected together. The geological range of the family is from the 

 Ordovician to the Carboniferous inclusive. 



The type-genus of this family is Actinocrinus itself, in which the calyx, 

 though very variable in shape (figs. 294-296), always possesses three 

 basals, which form a hexagon, and are united superiorly with the five 

 primary radials and the lowest of the anal interradials. There are three 

 cycles of radials, and the highest radials carry each a double series of 

 brachial plates, which support the variously divided arms. There are 

 three or more anal plates, of which the lowest (fig. 299, a) always rests 

 upon the basals directly. There is a variable number of interradials, 

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

 with calcareous plates, and the brachial grooves are continued beneath 



the vault thus formed, as so many 

 tunnels, to the central and con- 

 cealed mouth. The anus may or 

 may not be extended into a pro- 

 boscis, and it is sometimes very 

 excentric, sometimes subcentral. 

 It has been shown that in some 

 of the Actinocri7iidoz (as in forms 

 y-y — ,1— < < ? -, \ ' P*r^y I belonging to other families) there 

 XX) v~->< v ^)<^/~r^r\ ex i sts m the interior of the calyx 



a singular convoluted calcareous 

 plate, of a reticulated texture, 

 shaped somewhat like an ordin- 

 ary Bubble-shell (Bulla), occupy- 

 ing the vertical axis of the body, 

 and often of large size. This has 

 been compared with the calca- 

 reous structures present in the 

 " sand-canal " of various Echino- 

 derms ; but it is probably rather 

 an extreme development of the 

 discoidal calcareous plates which 

 have been described as strength- 

 ening the double wall of the spirally-twisted alimentary canal in the 

 living ComatulcF. The genus Actinocrinus appears to commence in 

 the Silurian, and is also represented in the Devonian; but it attains 

 its maximum in the Carboniferous, and is wholly unknown in later 

 deposits. Agaricocrinus and Batocrinus are Carboniferous forms very 

 closely allied to Actinocrinus. The Silurian genera Periechocrinus and 

 Megistocrinus are close allies of Actinocrinus, and the Carboniferous 

 Amphoracrinus and Dorycrinus only differ from it in comparatively 

 trifling particulars. 



In the Silurian and Devonian genus Melocrinus, there are four or 

 three basals, the lowest anal plate is separated from the basals by the 

 primary radials, and the arms are in the form of five free rays giving off 

 lateral armlets. Melocrinus is often regarded as the type of the separate 

 family of the Melocrinidce, to which Wachsmuth and Springer also refer 

 the genus Glyptocrinus, which will be briefly noticed later. 



Fig. 299. —Diagram of the dissected calyx of 

 Actinocrinus (after Schultze). b, Basals; r, 

 Radials ; i, Interradials ; a, The lowest of the 

 anal plates. 



