THE ECHINODERMATA 259 



4. Diploporita, in which the stereom-cavities are 

 normal to the surface and grouped in pairs, giving rise to 

 an appearance which must by no means be confused 

 with the very different " pore-pairs " of Echinoidea. 

 There is fair pentamerous symmetry, and the food- 

 grooves radiate on the surface of the theca. Spharonites 

 (Ord.) and Glyptosphcera (Fig. 73, c, Ord.) are spheroidal 

 contemporaries of Echinosphwra. 



The Blastoidea may be illustrated by Pentremites 

 pyriformis (Fig. 74), of the Lower Carboniferous of North 



FIG. 74. PENTREMITES PYRIFORMIS, SAY, MISSISSIPPIAN 

 (CHESTER GROUP), ALABAMA, U.S.A. 



a, Side view. (Natural size.) 6, Oral region. (X2.) (Original.) B, Basals ; 

 ft, radials ; L, lancet-plates ; O, orals or deltoids. In t>, the mouth is 

 in the centre, the five spiracles around it, and beyond and alternating 

 with these the five ambulacra. 



America. The shape suggests a flower-bud just opening. 

 There is no true stem, but the base is drawn out into a 

 stem-like stump. It would consist of five lozenge-shaped 

 basals, but two pairs are fused, so that only three basals 

 are counted. The five radials are much larger and would 

 be hexagonal but for a very deep notch which almost 

 cuts each in two. Five small orals or deltoids alternate 

 with the radials above. Between these, and in the 

 notches of the radials, lie five lancet -plates, each with a 

 central food-groove into which run numerous oblfque 



