94 LIFE ON THE EARTH. 



R. Wilsoni of the Silurians; R concinna of the Oolite; 

 R. octoplicata of the Chalk ; with R. psittacea of the Nor- 

 wegian Seas ; 



we shall perceive how very small is the amount of 

 change which all the lapse of time has witnessed in 

 the forms of what seem to be among the most varia- 

 ble as well as the most numerous of fossil shells. 



Rudista are not known below the Cretaceous 

 Series. 



Monomyaria. Classing these in four great fa- 

 milies, Aviculidee, Pectinidse, Limidse, and Ostreidse, 

 which are all found both fossil and recent, we remark 

 in the first place the constancy of the general charac- 

 ters of each. Thus Ostreidse shew always rudely 

 laminated shells ; PectinidsB and limidse are nearly 

 equivalvular, neat, and radiated; the former nearly 

 equilateral, the latter more oblique ; but in this 

 respect yielding to the Aviculidae, which are more 

 frequently smooth externally, and pearly within, and 

 have very unequal valves. No true oysters occur 

 below the Mesozoic strata. Pecten and Lima are 

 not known below the Carboniferous Limestone ; Avi- 

 culidoe belong to every geological age. 



Dimyaria. Two great divisions constitute this 

 large group of shells. To the Asiphonida, which ap- 

 proach nearest to Monomyaria, and include three 

 marine groups, Mytilidse, Arcadse, Trigoniadse, and one 



