1864.] Jzenuins on Brackish-water Fossils of Crete. 419 
fossils in question having been deposited in a lake or an estuary, and 
this discussion is the more desirable, because the more or less fresh- 
water formations of Asia Minor, &c., have often been treated of as 
necessarily lacustrine. The only circumstances necessary to remember 
are: (1) that the following remarks do not refer to Cerigo, the fossils 
from thence being normally estuarine ; and (2) that in Crete the most 
essentially salt-water genera are represented by species normal in 
character, while the fluviatile genera are represented by distorted 
species, 
But to enable us to decide whether we have been dealing with a 
marine, an estuarine, a fluviatile, or a lacustrine formation, it is now 
necessary to discuss the distinctive characters of these classes of 
deposits, chiefly from a paleontological point of view. 
Distinctive Characters of Lacustrine, Fluviatile and Estuarine 
Deposits—Purely fresh-water strata are nearly always lake-deposits, 
because a river seldom deposits in its own bed, and when it does, the 
deposit is so insignificant, that it is rarely preserved ; while, on the 
other hand, the deposit of a river at its mouth, that is, a delta, contains 
brackish-water shells, generally mixed with those of fluviatile and 
terrestrial origin. Again, a lake may be more or less brackish, or 
even absolutely salt ; and a lagoon, which is but another name for a 
lake connected with a larger body of water, may be subject to periodical 
irruptions of salt water. Thus there are many contingencies to be 
guarded against in deciding as to the lacustrine or estuarine origin of 
a series of beds, supposing the fossils contained in them to exhibit 
characters not antagonistic to the presence of a certain quantity of 
salt water, especially in the region under consideration, where lagoons 
are so abundant ; but very little difficulty exists if the shells happen 
to be purely freshwater and normal in character. Of course, there is 
this difference between a lake and a river, that whereas the water in 
the former is more or less stagnant, that in the latter is in motion; 
but a deposit from a river into a lake would yield evidence of both 
running and stagnant water, and, unfortunately, shells afford but 
little evidence as to their fluviatile or lacustrine origin. It would, 
however, be strange indeed if the fossils of a true lacustrine deposit 
did not consist, to a certain extent, of the shells of pulmoniferous 
mollusks ; and inasmuch as there is not a single shell belonging to 
that group amongst the fossils under consideration, the theory of a 
fresh-water lake cannot well be accepted. 
Nature of the Crete Deposit.—All the fossil genera under notice 
from Crete, excepting the genus Unio, have existing species which 
live in brackish water, or even in the sea, so that they are not antago- 
nistic to the estuarine nature of the deposit, though they are equally 
favourable to its being a salt-lake formation; but as some of the 
genera cannot exist in fresh water, the beds cannot have been 
deposited in a fresh-water lake. Again, Neritina and Melanopsis are 
essentially the inhabitants of running water, and the genus Unio is 
just as essentially fresh water, therefore if the fossils presented no 
abnormal characters, the only rational conclusion would be that the 
Crete formation is a deposit from a river in an estuary. 
