HEADON BEDS 



145 



downward into the Barton Clay but does not occur above the 

 Headon Series. Nucula headonensis is also very plentiful in 

 Colwell Bay. 



The estuarine and freshwater bivalves most commonly met with 

 are species of Potamomya (Fig. 39) and Ci/rena. These occur in 



Fig. 39. 



Potamomya plana^ Sow. 



vast numbers in certain beds. Unios (Fig. 40) are more rare and 

 are generally confined to thin seams. 



Fig. 40. 

 Unio Solandri, Sow. 



The most plentiful univalves in the marine and estuarine beds 

 are several species of Gerithium, including G. concavum (Fig. 41) 

 and G. pseudo-cinctum (Fig. 43), Melanopsis sid)fusiformis 

 (Fig. 42), Buccinum labiatum, Murex sexdentatits, Nerita aperta. 



Fig. 41. 



Cerithium 



concavum, Sow. 



Fig. 42. 



Melanopsis 



suhfusiformis, 



Morris. 



Fig. 43. 



Cerithium 

 pseudo-cinctum, D'Orb, 



Neritina concava, Ancilhiria huccinoides, Melania muricata, and 

 several species of Gancellaria, Natica, Pleurotoma, and Valuta. 

 E 56786. S 



