PLATE XX. 



Pygaster from the Cornbrash. 



Fig. 



1 a. Pygaster Morrisii, WrigJd, p. 280. Upper surface, natural size. 



b. Under surface, natural size. 



c. Lateral view, natural size. 



d. Ambulacral area, poriferous zones, and inter-ambulacral plates, magnified four 



diameters. 



e. Primary tubercle from tlie upper surface, with areolar circle of granules, magnified 



eight times. 

 /. Primary tubercle from the base, magnified eight times. 



Pygaster from the Coral Rag. 



2 a. Pygaster umbrella, Agassiz, p. 282. Upper surface, natural size. 



b. Under surface of another specimen, natural size. 



c. Lateral view of a. 



d. Ambulacral area, poriferous zones, and inter-ambulacral plates, magnified three 



diameters. 



e. A portion of the apical disc, showing the four perforated ovarial plates and madrepori- 



form body, magnified two and a half times. 

 /. A primary tubercle, with its areolar circle of granules, magnified eight times. 

 g. A primary tubercle from the base, magnified eight times. 



