C 253 



CHAPTER XL 



FOSSIL MOLLUSCA (MESOZOIC, OR SECONDARY). 



FOSSIL mollusca increase very rapidly, both in species 

 and numbers, as we explore the strata of the 

 Secondary formations. They are absent from the 

 Trias of Great Britain, the greater part of whose beds 

 seem to have been formed along the bottoms of large 

 lakes, something like the existing Dead Sea, whose 

 waters were too salt for Mollusca to live in. In the 

 Rhaetic beds, however, which overlie the upper Trias, 

 and which seem to have been formed under semi- 

 marine or brackish-water conditions, bivalves are very 

 abundant, and some of them are quite characteristic, 

 such as Cardium Rhaticum, Pecten Valoniensis, Avi- 

 cula contorta, Ostrea liassica. The student will find 

 them in any quantity in the rocks forming the bold 

 headland of Penarth, just beyond Cardiff, and also 

 at Aust Cliff, on the opposite side the Severn estuary 

 to Chepstow. At the latter place the grey Rhaetic 

 strata succeed the red Trias, and the ground is strewn 

 with fallen blocks of the former, where some splendid 



