302 GEOLOGY OF THE ISLE OF WIGHT. 



at Pounceford in Sussex.* It also occurs in 

 Hanover, in the clays intercalated with beds of 

 coal, belonging to the Wealden formation. 



Fossil freshwater bivalves. — Many of the 

 bluish grey argillaceous limestones, as we have 

 before remarked, are chiefly composed of five or 

 six species of bivalves of the genera Gyclas and 

 Cyrena ; a small slab with several shells in relief, 

 is represented PL VI. fig. 4. Large blocks covered 

 with shells of this kind, are commonly to be found 

 along the base of the cliffs, at Brook, Compton, 

 and Sandown bays ; and in the clays, there are 

 numerous seams of crushed shells of these genera. 

 The hard grits often abound in casts of Cyclades, 

 associated with a small species of river-mussel 

 ( Unio) ; and some layers of Sussex marble contain 

 an intermixture of these shells with the paludinae. 



Unio Valdensis. PL VI. fig. 1. — The only 

 other bivalves which occur in any considerable 

 number in the Wealden, are the mussels termed 

 Unio?iidce ; a family of testaceous mollusca contain- 

 ing several genera, and comprising many species 

 of considerable magnitude. These animals have a 

 solid pearly shell, with two principal and two 

 Lateral teeth on the hinge. Tn number, variety, 

 and internal lustre, the Uniones of the large 



• Wonik-rs of Geology, j>. 350 ; it is there named Melanopsii. 



