ECHINODERMATA. CRINOIDEA. 89 



plates over the upper surface of the calyx, hiding the 

 grooves and the mouth. This roof is in some genera 

 provided with a conical process, the anal proboscis, at the 

 summit of which is the anus. In these forms, as well as 

 in a few others, the grooves in the arms are also roofed 

 over with plates. 



There are two orders, (1) Palseocrinoidea, (2) Neocri- 

 noidea. 



ORDER. PAL^OCRINOIDEA. 



The calyx is large and the arms small, relatively. 

 Inter-radials are generally present. On the ventral surface 

 there is a roof of plates hiding the food grooves and the 

 mouth. 



Actinocrinus. Calyx variable in form, pear-shaped, 

 ovoid, or more or less spherical ; monocyclic. Basals three, 

 forming a hexagon. Three cycles of radials firmly united, 

 the first high, the third bearing the double series of 

 brachials. Inter-radials present and also the anal inter- 

 radial, the first plate of the latter resting on the basals. 

 The ventral roof formed of thick, tubercled, hexagonal 

 plates ; anal proboscis sometimes present. Arms ten to 

 thirty, with two rows of plates. Stem circular, canal 

 pentagonal. Silurian to Carboniferous Limestone. 



Crotalocrinus. Dicyclic. Underbasals five, small, 

 pentagonal. Basals very large. One cycle of radials, very 

 large. Inter-radials absent, except a small anal inter-radial. 

 Arms five, with a single row of plates ; dichotomous, the 

 branches uniting so as to form lamellar expansions or 

 networks, which are enrolled. Ventral roof formed of 

 small plates ; in the centre five large oral plates. Anal 

 proboscis marginal. Stem thick, circular; canal penta- 

 gonal ; roots thick. Wenlock Limestone. 



