ECHINOCONID^. 199 



The posterior ovarial is often replaced by a complementary, imperforate plate, which 

 is sometimes wanting altogether. 



The spines are small, short, and subulate. 



The EcHiNocoNiDyE are distinguished from the Echinobrissid.e, Echinolampid^, 

 and Clypeasterid^, by their simple poriferous zones ; in this respect the family 

 resembles the Echinonid^, from which, however, it differs in possessing a peristome 

 furnished with auricles and a masticating apparatus ; the form is likewise more circular 

 and elevated, and the tubercles are more developed and arranged in longitudinal rows. 



The EchinoconiDjE are an extinct family, found only in the Oolitic and Cretaceous 

 rocks; at present we recognise six well-defined genera in this natural group, which 

 present the following opposite characters : 



Test elevated ; ambitus with projecting internal septa ; vent 



inferior always ..... Discoidea. 



Test elevated ; peristome sub-decagonal ; vent marginal . Echinoconus. 



Test depressed ; ambitus without internal septa ; vent inferior 



and marginal ..... Holectypus. 



Test depressed ; peristome decagonal ; vent oblique, distant from 



the disc ; tubercles perforated and crenulated , . Anorthopygus. 



Test depressed ; peristome decagonal ; vent pyriform, separated 

 from the disc; tubercles perforated, but not crenulated; 

 irregularly superposed pores . . . Pileus. • 



Test depressed ; peristome decagonal; vent pyriform, not separated 

 from the disc ; tubercles perforated and not crenulated ; pores 

 xmigeminal, regularly superposed . . . Pygaster. 



I. Gemis — Discoidea, Klein, 1734. 



Galekites, pars, Lamarck, 1801. Discoidea, Gray, 1834. Discoidea, Agassiz, 1836. 



Discoidea, Desor, 1842. 



Test circular or subpentagonal at the border ; upper surface much elevated, hemi- 

 spherical ; sides vertical or slightly convex ; inferior surface flat, slightly concave. 



Poriferous zones very narrow, and converging in a straight line from the disc to the 

 peristome. 



