278 CPvINOIDEA. 



development of the discoidal calcareous plates, which have 

 been described as strengthening the double wall of the spir- 

 ally-twisted alimentary canal in the living Comatula. The 

 genus Actinocrinus appears to commence in the Upper Silu- 

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

 its maximum in the Carboniferous, and is wholly unknown 

 in later deposits. Megistocrinus, Agaricocrinus, and Batocrinus 

 are Carboniferous forms very closely allied to Actinocrinus, 

 and perhaps of but sub-generic value. The Silurian Perie- 

 chocrinus is another close ally of Actinocrinus, and the Car- 

 boniferous Amphoracrinus and Dorycrinus only differ from it 

 in comparatively trifling particulars. 



In the Devonian genus Melocrinus, though the structure 

 is in many respects similar to that of Actinocrinus, there are 

 four basals, and the lowest anal plate is separated from the 

 basals by the primary radials. 



In the family of the Platycrinidce, as typified by Platy- 

 crinus itself (fig. 152), the cup consists of three basals (as 

 in Actinocrinus), which support two cycles of radial plates, 

 of which the primary ones are much the largest. The 

 secondary radials carry the numerous bifurcated arms, all 

 the divisions of which bear pinnulse. There is a single inter- 

 radial in each of the inter-radial spaces, and there may be 

 one large or three small anal plates. The column is rounded 

 near the calyx, but its lower joints are oval and compressed. 

 There is, typically, a large anal proboscis. In connection 

 with the proboscis of Platycrinus, we may just notice the 

 well-known fact that in many specimens (as is the case 

 with other Crinoids possessing a similar elongated anal 

 tube) there is found in close apposition with the proboscis, 

 and often placed upon its actual summit, the shell of a fossil 

 Univalve (apparently almost always, or always, a species of 

 Platyceras). It was originally supposed that the Crinoid 

 had been fossilised in the act of eating the Mollusc the 

 anal tube being regarded as the mouth but all the living 

 Crinoids feed upon microscopic animalcules, and this sup- 

 position is therefore, prima facie, an improbable one. It has 

 also been shown by Meek and Worthen that the Platyceras 

 must have lived for a long time attached to the proboscis of 



