ECHINODERMATA. BLASTOIDEA. 97 



not visible externally, but is covered over by a roof of 

 small plates. Arms are represented by pinnules only, 

 which are given off from the margins of the ambulacral 

 areas. 



The other blastoids differ from the one described 

 mainly in the relative sizes of the plates, in the distance 

 which the ambulacra go towards the base, and in the 

 amount of folding of the hydrospires. 



Granatocrinus. Calyx elliptical, oval, or more or 

 less spherical, in section pentagonal or round. Basals 

 small, not seen in a side view. Radials of variable size. 

 Deltoids generally rhombic, large in some species, small 

 in others. Ambulacra narrow, straight. Lancet-plate 

 narrow. Hydrospires simple, often not folded, dilated at 

 the end. Spiracles round or oval, piercing the apices of 

 the deltoids, the posterior one including the anus. Car- 

 boniferous Limestone. 



Distribution of the Blastoidea. 



In England the Blastoids range from the Devonian to 

 the Carboniferous, being most abundant in the latter. 

 But in North America they are also found in the Silurian. 

 The English Devonian forms are rare and but little 

 known. In the Carboniferous Limestone the blastoids 

 attain their maximum, ten genera are represented, the 

 most important being Granatocrinus and Codaster. Pen- 

 tremites does not occur in Britain. 



w. p. 



