240 Proceedings of the Royal Dublin 8ociely. 



animals (invertebrates) is connected with, tlie great movements 

 wliioh have affected the earth's crust. The earliest "well-known 

 lacnstrine areas are those of the Old Red Sandstone, the direct de- 

 scendants of the preceding Silurian seas. In the deposits of one 

 of these lakes we meet with the earliest known freshwater mollusc, 

 Anodonta Jukesii (Forbes). Probably, considering the remoteness 

 of the alliance of Lymncea to existing marine genera, it also, 

 and, perhaps, Valvata with it, originated at the same time as 

 Anodonta. 



The lakes of the Permo-Triassic period, the residues of car- 

 boniferous seas, produced considerable additions to the freshwater 

 fauna of the globe. The Neritidce and CerithiadcB are probably 

 post-Palaeozoic families, and as the Neritidce and Melaniidce are 

 so closely connected with them, they may be regarded as col- 

 lateral, or more probably direct descendants, and thus may have 

 originated in Triassic lakes, but not earlier. Other genera pro- 

 Imbly arose at the same time ; the occurrence in Cretaceous deposits 

 of Unio, Physa, Valvata, and Lymncea in the Nearctic, Paleearctic, 

 and Oriental regions, suggest a high antiquity for these genera; 

 but they may have existed, as we have suggested, in Palseozoic 

 times. 



The lakes of the Tertiary period furnished probably further 

 contributions to our freshwater fauna, such as Lithoglyphus and 

 Dreissena. Thus, existing freshwater genera are probably descended 

 from marine forms which became metamorphosed in the waters of 

 the Devonian, Triassic, and Tertiary lakes. In the lakes of Central 

 Africa the tertiary freshwater fauna still survives ; nearly all of 

 the genera from Lake Tanganyka, described by Edgar Smith, 

 being referable to genera already in existence in Mesozoic and 

 Tertiary times. The lakes of the Northern Hemisphere received, 

 on subsiding beneath the glacial sea, such arctic forms as Mysis 

 relicta ajid Pontoporeia affinis,hvii most of their existing inhabitants 

 have re-entered them, since their emergence from the sea, from 

 their tributary rivers, or the rivers of adjacent regions. 



