404 RETROSPECT OF THE OOLITIC SYSTEM. CHAP. 



AFFINITIES OF OOLITIC PHOLADOMY^}. 



gigantea. 



Candida. 



margaritacea. 



decussata. 



Tliis group, so abundant in the oolites, begins probably in the 

 lower lias with Pholadomya ambigua, Sowerby, and passes with 

 easy modifications to the cretaceous system, and thence through 

 analogous tertiary forms to the living species P. Candida, a rare 

 West Indian shell. This 'main-line' is flanked by a parallel 

 through P. acuticosta to P. rustica and gigantea, and by another 

 through P. fidicula to P. insequalis, both families apparently be- 

 coming extinct with that oolitic kingdom which gave them their 

 earliest homes. 



