94 



ECHINODERMATA. CYSTIDEA. 



Distribution of the 

 The Cystideans are comparatively rare fossils, they 

 range from the Menevian Beds to the Carboniferous 

 Limestone, and attain their maximum development in the 

 Upper Ordovician. In the Menevian we get Protocystites ; 

 this also occurs in the Tremadoc, and with it Macrocystella. 

 In the Upper Ordovician we get amongst others, Echino- 

 sphcerites, Ateleocystites, and Pleurocystites ; in the Silu- 

 rian Lepadocrinus and Ateleocystites. In the Devonian 

 there are fewer, and in the Carboniferous only one or two 

 genera are known. 



CLASS. BLASTOIDEA. 



In the Blastoids, as in the cystideans and many of 

 the crinoids, the body consists of a stem and calyx, but 

 the former is rarely found attached. The calyx may be 

 spherical, oval, pear-shaped, or bud-like ; it is formed 

 chiefly of thirteen plates, arranged in a regular manner. 

 True arms are not present. 



In a typical form (Pentremites) the calyx (fig. 24) has 



BC 



b b an 



FIG. 24. Pentremites godoni, Carboniferous. A, lateral; B, upper sur- 

 face; <7, under surface, a, ambulacral areas; b, basal plates; r, 

 radials ; d, deltoids ; s, spiracles around the mouth ; an, anus. 

 (Twice natural size.) 



