66 GUIDE TO THE FOSSIL INVEETEBEATE ANIMALS. 



Brachioles 



Deltoids 



Radials 



Basal s 



Theca 



G-allery former are among the Bohemian and French Cystids, the 

 last is well shown among the British fossils. 



Class BLASTOIDEA. 



Table-case The Blastoids (Bud-shaped) form a small class of Echino- 

 derma, which arose at an early period, probably from Cystidea 



Diploporita, and flour- 

 ished chiefly in Devo- 

 nian and Carboni- 

 ferous times. The 

 Museum possesses a 

 rich collection, which 

 served as basis of a 

 monograph wiitten by 

 Etheridge and Carpen- 

 ter and published by 

 the Trustees. A list 

 of the specimens has 

 also been issued. 

 Since the fossils are 

 too small to be seen 

 clearly, only a few 

 characteristic exam- 

 ])les are exhibited. 

 The general appear- 

 ance of the blastoid 

 skeleton and the terms 

 applied to its more 

 obvious parts are 

 shown in Fig. 30. The 

 brachioles border five 

 food-grooves, of which 

 the skeleton is rather 

 complicated. The con- 

 tiguous edges of the 

 plates termed deltoids 

 and radials were 

 folded, and in most of 

 the genera these folds 

 projected far into the 

 interior of the theca 

 and thus enabled the 



Fig. 30.-A typical Blastoid, Orophocrimis fusi- ^g^ated sea-water to 

 ^r^^s, Carboniferous (Kmderhook) of Iowa, ^^^^^ ^^^^^ 



Stem 



Root 



