ix PHYLUM ECHINODERMATA 405 



SUB-CLASS I. MONOCYCLICA. 



Crinoidea in which the base of the theca comprises basals 

 only. 



SUB-CLASS II. DICYCLICA. 



Crinoidea in which the base comprises basals and infra-basals. 



CLASS II.- CYSTOIDEA. 



Fixed, stalked, or sessile Pelmatozoa, with the plates of the theca sometimes 

 irregular, sometimes arranged in a regular radial system, with food-grooves 

 extending for a longer or shorter distance over the surface of the theca, some- 

 times on special plates lying above those of the latter, their terminal parts 

 extending on to a varying number of unbranched arms or " fingers " ; the 

 theca perforated completely or partially by numerous pores which are supposed 

 to have lodged respiratory processes. 



Lower Silurian to Carboniferous. 



CLASS III.-BLASTOIDEA. 



Fixed Pelmatozoa with well-developed stalk, and theca with a regular 

 system of plates ; with five, rarely four, food-grooves radiating out from the 

 central mouth, and each borne on a special " lancet plate," the inter-radial 

 intervals between which are occupied by a corresponding number of oral or 

 " deltoid " plates. The grooves are bordered by a series of side plates bearing 

 small branches or " fingers " to which side branches of the grooves extend. In 

 the intervals between the grooves on the aboral sides of the deltoids are a whorl 

 of plates psrf orated by the apertures of groups of internally situated respiratory 

 folds (hijdrospires). The anus is eccentrically situated on the oral surface. 



Upper Silurian to Carboniferous. 



CLASS IV.-EDRIASTEROIDEA. 



Fixed (or sometimes free ?) Pelmatozoa, usually sessile, rarely with a short 

 stalk ; with sac-like, cushion-shaped or disc-shaped theca made up of numerous 

 plates devoid of any regular arrangement and without any appendages ; with 

 central mouth and five straight or curved radiating food-grooves bordered by 

 covering plates : anus and rnadreporite on oral side. 



Cambrian to Carboniferous. 



CLASS V.- CARPOIDEA. 



Pelmatozoa with a well-developed stalk, with the body laterally compressed, 

 with only two food-grooves running out from the mouth. Theca composed of 

 numerous small irregular plates with larger lateral plates forming a framework 

 along the margins. 



Cambrian and Silurian. 



Systematic Position of the Examples. 



Asterias rubens is a species of the genus Asterias, which, with 

 several others, constitutes the family Asteriidce of the order 

 Forcipulata. The family Asteriidce is characterised among the 

 families of the Forcipulata by the following distinctive features : 

 The ossicles of the aboral surface are small, unequal, reticulate 

 plates, bearing isolated or grouped spinelets (paxillce). The margin 

 of the actinostome ie defined by the ambulacra! plates. The 



