Sottas—On the Origin of Freshwater Faunas. 105 
for we can easily find close alliances for them amongst marine molluscan families 
from which we may regard them as directly or collaterally descended. Further, 
as fossil remains of marine molluscs are comparatively abundant, we shall be able 
from the first appearance of these to gather some idea as to the probable date of the 
first appearance of their freshwater relations ; always, however, bearing in mind that 
the determinations of many Palzeozoic Mollusca are to a certain extent doubtful. 
Setting these aside, we find that the Cyprinidze (in sensu restricto) date from the Trias, 
and thus Cyrena, which is allied to this family, in all probability may be regarded 
as post-Triassic in age. The Neritinz are included in the family Neritide, which 
is also first found in the Trias, while Hydrobia belongs to the Rissoide, first met 
with in the Jura, though it may of course have made its appearance earlier, and 
probably did. Thus the genera Cyrena, Neritina, and Hydrobia cannot safely be 
pushed farther back than the Trias, but may very possibly have then originated 
in the lakes of that period. 
Passing now to the Purbeck-Wealden deposits, we encounter the numerous 
freshwater genera detailed in the following list :—Unio, Cyrena, Corbula, Cardium, 
Valvata, Hydrobia, Amnicola, Neritina, Planorbis, Lioplax, Bithynia, Paludina (?), 
Physa, Limnzus, Gnathodon, Pleuroceras, Goniobasis, Leptoxis, Plychostylus, 
Carychium, and Auricula. Of the genera which here appear for the first time, Gna- 
thodon, a sub-genus of Mactra, may have become specially modified in Jurassic 
times, for the genus is not known earlier than the Coral-rag. Unio, however, may 
be regarded as a descendant of the Devonian Anodonta. The list is more espe- 
cially interesting as affording evidence of the abundant development of the fresh- 
water Melaniine in our own area during the Mesozoic period, from which, as well as 
from Northern Europe generally, they are now entirely absent, though widely spread 
in other regions, particularly the warmer zones of the earth. Considering the small 
part of the earth’s crust that has been at all carefully studied, it is of the highest 
interest to find the same genera of freshwater shells occurring at widely-separated 
points in beds of approximately contemporaneous age, and so far remote from the 
present as Cretaceous times (Cenomanian or Senonian). Thus in North America, 
where the Cretaceous rocks have been grouped as follows:—Laramie, Fox Hill, 
Colorado, and Dakota groups, they have afforded Unio (one species), Cyrena (one 
species), Neritina (three species), Physa, and Valvata, from the Fox Hill beds; 
and Unio, Corbicula, Acella, Leptolimnea, Limnophysa, Limnza, Neritina, Mela- 
nopsis, Campelosis, Pyrgulifera, and others, from the Laramie beds." With regard 
to the fauna of the Fox Hill beds, Clarence King well remarks that ‘the disco- 
very of this singularly tertiary-like group deep in the Cretaceous should only 
open our eyes to the early specialisation of freshwater molluscan types.” And 
Dr. White, referring to the Laramie fossils and their similarity to existing forms, 
speaks more strongly still, as when he says: ‘‘'The lines of descent of the nume- 
rous types which have reached us unbroken seem to be almost De so little 
