112 LIFE ON THE EARTH. 



valves, through the vast series of deposits which ex- 

 tends from the Devonian rocks to the present ocean 

 similar forms, similar accompaniments, and similar 

 exclusions of other forms, such as Brachiopoda, 

 Monomyaria, &c. So that the law of segregation (if 

 we may use this term) which separates the few 

 freshwater from the many marine races, operates 

 now as it did operate in the Palaeozoic ages, and 

 similar races are subjected to its influence. During 

 all this time Unionidse have maintained their cha- 

 racteristic structure and associations. Taking next 

 the Gasteropoda we remark in living nature two 

 groups,- one separating air from water by means of 

 gills like fishes, the other breathing air by lungs. 

 Of the former, the living examples cluster round the 

 genera, Paludina, Yalvata, Ancylus, Neritina, the 

 former are represented by Planorbis, Limnsea, Physa, 

 Melania. In a fossil state these forms are recog- 

 nized at several stages, but not at so early a period 

 as the Dimyarians. They are not known to occur 

 below the Purbeck Strata. 



Csenozoic . . . Paludina, Neritina, Planorbis, Physa, Limnsea, Me- 

 lania, &c. several zones in the Fluvio-marines 

 of the Isle of Wight. 



"Wealden ...Paludina, Melania, Melanopsis. 



Purbeck ...Paludina, Valvata, Melania, Physa, Limiisea, 

 Planorbis, 



