182 Dr. T. Wright on the Cassidulidie of the Oolites. 



ovarial; the madreporiform body is generally situated in the 

 centre, around which the ovarials arc arranged ; the five ocular 

 plates are small, and lodged at the apices of the ambulacra. 

 From the structure of the ambulacra we divide the Cassidulida? 

 into two groups. 



1st Group of Echinoneides*. 



Ambulacra simple, uniform throughout ; mouth circular, deca- 

 gonal or pentagonal, without prominent lobes. 



Genus Pygaster, Agassiz. 



Test subpentagonal, more or less depressed ; surface of the 

 plates covered with small perforated tubercles, raised on mammil- 

 lated and crenulated eminences, disposed in regular vertical 

 rows, and surrounded by areolae with encircling granules ; the 

 tubercles attain their greatest development at the circumference 

 and base of the test ; ambulacral arese with four or six rows of 

 tubercles ; interambulacral area? with from 12-20 rows ; mouth- 

 opening central and circular, margin divided into ten equal 

 lobes, the notches of which correspond to the sutural junction of 

 the ambulacra with the interambulacra ; anal opening very large, 

 occupying the upper half of the single interambulacral area ; 

 pores arranged in simple pairs throughout the avenues ; apical 

 disc unknown. This genus is extinct ; the species are distributed 

 throughout the Oolitic and Cretaceous rocks. 



Pygaster semisulcatus ? Phillips. 



Syn. Galerites umbrella ?, Lam. Anim. sans Vert. t. iii. p. 25. 

 Nucleolites umbrella, Defrance, Diet. Sc. Nat. t. xviii. p. 87. 

 Clypeus semisulcatus, Phillips, Geol. of York, part 1. pi. 3. fig. 17. 

 Clypeus ornatus, Buck. Murchison's Geol. of Cheltenham, 2nd ed. 

 p. 95. 



Test subpentagonal, depressed; interambulacra with from 16-18 

 rows of tubercles at the circumference ; ambulacra prominent 

 and convex, with four rows of tubercles ; anus very large, oc- 

 cupying nearly the upper half of the single interambulacrum ; 

 mouth deeply notched ; spines short, subulate, and longitudi- 

 nally striated. 



Height 1 inch and T 5 ^ths, antero-posterior diameter 3 inches, 

 transverse diameter 3 inches. 



Description. — The imperfect condition of the test of Pygaster 



* The group of Echinoneides comprehends eleven genera : Echinoneus, 

 Van Phels; Pygaster, Agass. ; Holectypus, Desor ; Discoidea, Gray ; Gale- 

 rites, Lam. ; Pyrina, Desml. ; Globator, Agass. ; Caratomus, Agass. ; Nu- 

 clenpyguSy Agass. ; Hyboclypus, Agass. ; Dysaster, Agass. 



