79 



area and delimited on its inner aspect by an ill-defined obtuse 

 ridge extending from the umbo to the post-ventral extremity. 

 The inner margin of the valves is smooth 

 (at any rate visually so) at post-ventral ex- 

 tremity. The ornamentation consists of 

 subacute concentric undulations of variable 

 strength and at variable distances, separated 

 by shallow concave spaces wider than the 

 ridges ; coincident with the undulations are 

 widely-separated linear growth - lines ; the 

 concentric undulations are continued beyond 

 the post-umbonal ridge as close-set growth- 

 lines. 



D imensions. — Umbonal- ventral diameter, 

 18 ; antero-posterior diameter, 22 (by estimate) ; transverse 

 diameter, greatest at about one-third from the unbo, 9 mm. 



Habitat. — One specimen of two valves in apposition, the test 

 replaced by Precious Opal ; the interior matrix not opalized. 

 Upper Cretaceous at White Cliffs, New South Wales; the 

 unique type in the Museum of the School of Mines, Adelaide. 



