FOSSIL MOLLUSC A. 267 



Inoceramus sulcatus and Inoceramus concentricus are 

 also abundant forms, and they often serve to distin- 

 guish a block of Lower White Chalk from the Upper, 

 where they are either uncommon or absent. 



In the Chalk one of the commonest and most 

 beautiful fossils is the bivalve Spondylus spinosa, which 



Fig. 246. Pecten interstriatus (Greensand). 



also rejoices in the other names of Plagiostoma spinosa 

 and Lima spinosa, much to the unnecessary bewilder- 

 ment of the student. It is difficult to extract it with 

 its long spines intact, but patience, and the fortunate 

 softness of the moist chalk when the fossil is first 

 extracted, as well as the clever use of a camel's-hair 



