DEVONIAN LAKES 195 



always threatening to carry them to sea ; and further, for 

 ensuring their dispersion over extensive freshwater areas. 

 We may now return to an inquiry which we suspended 

 when we set out to search for this result. We had 

 enumerated three conceivable ways by which marine 

 animals might enter the freshwater world ; we abstained 

 from adding a fourth, for that marine animals may be 

 carried by birds or other organisms or winds, and plumped 

 down into rivers and lakes, there to assume a freshwater 

 existence, does not appear probable to us, however much 

 it may to others. Our first step will be, then, to inquire 

 into the evidence afforded by geology as to the conversion 

 of marine areas into lakes. The first recognised existence 



FIG. 67. Further Development of Peneus. 

 The my sis stage. Magnified 20 

 diameters. After Fritz Miiller. 



of lakes occurs in the Devonian period. The previously- 

 existing Silurian sea became gradually differentiated into 

 a northern region of lakes, which have left their sedi- 

 ments as the Old Bed Sandstone, and a southern region 

 which retained its marine character. The transformation 

 took place under circumstances which afforded an oppor- 

 tunity for the contemporaneous development of a fresh- 

 water fauna. The lakes were possibly not all fresh, some 

 may have been salt, but it is certainly significant that in 

 the Old Bed Sandstone of Ireland we know of one fossil, 

 far from rare at Kiltorcan, which was pronounced by no 

 less an authority than Edward Forbes to be a genuine 

 ancestor of the existing pond mussel. This shell, now 

 known as Palceanodonta Jukesii, is altogether different in 



