104 So~Las—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 
Jukesu 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 Salmonidz 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 Unionide 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 Limnzide: their world-wide distribution, and the difficulty of assigning them 
any close alliance with marine forms, suggests a high antiquity for the family. 
The Valvatide, so curiously distinguished by the persistence of an archaic 
character in their gill plumes, might also have inhahited the Old Red Sandstone 
rivers and lakes. No one supposed that the terrestrial gasteropods had originated 
already in the Paleozoic period, yet the discovery of Pupa and Conulites (Helix) 
in the coal-measures leaves no doubt on this point. 
The Helicidz are amongst the nearest allies, and are possibly ancestors, of the 
Limneeide, and the latter may be fairly looked for in strata of corresponding age. 
It must here be added that the Helicidz, 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 we 
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, Neritina, 
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 ; 
