132 



OUR COMMON BRITISH FOSSILS. 



The upper part of the skin of such star-fishes as 

 the "five-fingers" (Ur aster rubens) is thickened and 

 roughened by the presence of grains or irregular 

 spicules of carbonate of lime. If these grains had 

 gone on increasing in size by addition to their margins, 

 they would have grown until they touched each 



Fig. -Lo^.Pentremites 

 florcalis, one of the 

 Blastoidea, from Car- 

 boniferous limestone : 

 a, profile ; b, summit ; 

 c, base or pelvis. 



Fig. 109. Asterias tessetlata, one of the recent 

 Cushion-stars. 



other, but would not have fused, and then we should 

 have had regular plates instead of grains, and the 

 whole body would have been covered by a kind of 

 tessellated pavement. This is how the arms of the 

 brittle-stars (Ophiuridea) and the margins of the arms 

 and body of the cushion-stars (Goniaster and Asterias) 



