414 ZOOLOGY sect. 



extending on to a varying number of unbranched aims 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 perforated by the apertures of groups of internally situated respiratory 

 folds (hydroxpires). 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 madreporite 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 ruoens is a species of the genus Astcrias, which, with 

 several others, constitutes the family Asteriidm of the order 

 Gryptozonia. The family Asteriidm is characterised among the 

 families of the Gryptozonia by the following distinctive features : — 

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

 plates, bearing isolated or grouped spinelets {paxillm). The margin 

 of the actinostome is denned by the ambulacral plates. The 

 pedicellariae are of two forms, forceps-like and scissors-like. The 

 tube-feet are in four rows. Asterias differs from the other genera 

 of the family in having well-developed reticulate dorsal ossicles 

 bearing definite spines. 



The Sea-urchins of w T hich a short description has been given 

 are the genera Strongylocentrotus and Echinus, but the description 

 is sufficiently general to apply to any member of the family 

 Echinidce, to which these genera, with a number of others, belong. 



