146 ECHINODERMA. CRINOIDEA 



in the Carboniferous, A ctinocrinus, Amphoracrinas^Poterio- 

 crinus, Platycrinus, Rhodocrinus, and Woodocrinus. Crinoids 

 are rare in the Permian. Throughout the Mesozoic form- 

 ations they are much less abundant than in the Palaeozoic ; 

 in the Trias the characteristic form is Encrinus. In the 

 Jurassic, Antedon, Isocrinus, Pentacrinus, Saccocoma, 

 Apiocrinus, and Millericrinus are found, the first two living 

 on to the present day. In the Cretaceous the chief forms 

 are Marsupites, Uintacrinus, and Bourgueticrinus. In the 

 Cainozoic, crinoids are very rare. 



CLASS II. CYSTIDEA 



The stem in the Cystideans is short, and in some cases 

 absent. The calyx is usually more or less spherical or 

 ovoid, and varies considerably in structure in different 

 types; frequently the plates are very numerous, without 

 radial symmetry, and perforated by numerous canals ; 

 food-grooves usually extend from the mouth over the 

 surface of the calyx, and bear simple arm-like structures, 

 called brachioles (fig. 60). 



In Glyptosphwra (fig. 59 A), from the Ordovician, the 

 calyx is spherical, and composed of a very large number of 

 polygonal plates, which are without any radial arrange- 

 ment such as occurs in the Crinoids and Blastoids. The 

 mouth is at the summit of the calyx, and is covered by five 

 oral plates (a), between which the five food-grooves start 

 and extend in a radial manner over the upper part of the 

 calyx, sometimes giving off branches (b) ; at the ends of 

 these grooves are facets (c) to which the brachioles were 

 articulated. The grooves were protected by small covering- 

 plates. On one side of the calyx is the anus (d), which in 

 perfect specimens is covered by a pyramid of small 



