104 SOLLAS On the Origin of Freshwater Faunas. 



described lamellibranchs of the marine Devonian strata, and, from its large size, 

 rivalling that of existing Anodons, appears to have flourished under remarkably 

 favourable circumstances. If at this early period a glochidian stage was then 

 characteristic of the Anodonta, it would have secured a wide distribution to the 

 species by means of the contemporaneously existing fish. Although Anodonta 

 Jukesii is the only fossil found in the Old Red Sandstone which has been referred 

 to the freshwater Mollusca, others may undoubtedly have existed. Indeed the 

 presence of numerous remains of fish which are supposed to have possessed the 

 habits of the Salmonidse directly suggests this, since, judging from their teeth, they 

 must have required animals to feed upon during their sojourn in fresh water. 



The world-wide distribution of the Unionidse and their extraordinary richness 

 in subgenera and species are quite in harmony with this early appearance of 

 Anodonta ; and I feel disposed to assign quite as early a date to the appearance of 

 the Limnseidae : their world- wide distribution, and the difficulty of assigning them 

 any close alliance with marine forms, suggests a high antiquity for the family. 

 The Valvatidae, so curiously distinguished by the persistence of an archaic 

 character in their gill plumes, might also have inhabited the Old Red Sandstone 

 rivers and lakes. No one supposed that the terrestrial gasteropods had originated 

 already in the Palaeozoic period, yet the discovery of Pupa and Conulites (Helix) 

 in the coal-measures leaves no doubt on this point. 



The Helicidse are amongst the nearest allies, and are possibly ancestors, of the 

 Limnfeidse, and the latter may be fairly looked for in strata of corresponding age. 

 It must here be added that the Helicidse, though at least as old as the Car- 

 boniferous period, may be much older, and may have existed in the Devonian 

 forests, so similar in the general character of their flora to those of the succeeding 

 Carboniferous period. 



The next great lacustrine epoch occurred in Permo-Triassic times, when 

 extensive lakes covered a large part of the northern continental areas. Some of 

 these were evidently inland salt seas, but probably not all : indeed Ramsay regards 

 the Bunter beds of the Trias as in all probability the deposits of a freshwater lake, 

 which subsequently became salt. In such freshwater lakes a part of the enclosed 

 post-carboniferous fauna may have slowly become modified, and thus have contri- 

 buted additional genera to our freshwater fauna. The Trias has not, however, as 

 yet furnished us with any fossiliferous freshwater deposits, and consequently AVU 

 must look to later Mesozoic strata for signs of the freshwater genera which had 

 thus early come into existence. But scant evidence is however afforded until we 

 reach the Purbeck and Neocomian strata : thus in the Lias we find Cyrena, Neritma, 

 and, according to Moore, Planorbis (one species) and Valvata (two species). In the 

 inferior Oolite we meet with Corbula, Neritina, Planorbis, Paludina (?), Melania, 

 Hydrobia, and Cyrena. Of these genera, Planorbis, Paludina, and Valvata may be, 

 and probably are, very ancient forms which originated in Devonian lakes ; but with 

 the other genera Cyrena, Neritina, Melania, and Hydrobia the case is different ; 



