RUPERT JONES ON THE FOSSIL ESTHEEL2B. 
269 
that the recent Estherice appear, as it were, suddenly (like the Apus) 
in pools and ditches of rain-water, and are quickly developed in tanks 
and ponds dry for even ten or eleven months in the year, it is not 
unlikely that pools of fresh water, temporarily formed on a flat sea¬ 
shore, may have been inhabited by Estherice , destined to be quickly 
buried in the first wind-drift of sand, or at the return of high tides. 
As an inhabitant of brackish water, the Estherice would be still more 
likely to have been occasionally accompanied by marine shells: nor 
can we say that the fossil associates quoted above were not inhabi¬ 
tants of brackish water, or of salt lakes ; for experience is the only 
guide to the naturalist in determining whether the members of many 
of the molluscan groups affect marine, brackish, or freshwater habitats. 
Perhaps some might like to think that at first marine conditions 
alone suited aquatic animals, and that some have subsequently taken 
to brackish and freshwater habitats ; and this may have been the case 
with Estheria : but, except for the “progressive” aspect of the 
argument, the converse might just as well hold good for the Lingula , # 
Spirorbis, Avicula, Anthracosia, Anthracomya, and Eleurophorus, men¬ 
tioned as being found in the older rocks in company with Estheria. 
Of the living molluscan genera that are known to have fluviatile 
as well as marine species, the following are the most prominent:— 
JRissoa ( Assiminia ), Cerithium (Lot amides), Area ( Scaphula ), Sole cur - 
tus ( Novaculina ), Mytilus (Ereissena ), and Cardium, f but how the 
above, as well as Modiolce, See. (‘ Notices of the Royal Institution of Great Britain,’ 
vol. i. p. 285.) 
* Lingula tenuissima of the Trias bears evidence of its having been subjected 
to the gradually increasing influence of some deteriorating agency, probably fresh 
water; for in the several strata, from the Muschelkalk upwards, in Wiirtemberg, 
&c., Lingula are found to become smaller and smaller until Estheria rninuta 
comes in. 
f In Dr. J. E. Gray’s “ Memoir on Testaceous Mulluscs,” in the Philos. Transact, 
for 1835, he treats of “ Species of Testaceous Mollusca living in very different situa¬ 
tions from the majority of the known species of the genus to which they belong, or hav¬ 
ing the faculty of maintaining their existence in several different situationsand he 
illustrates the case (1st) of species of the same genus being found in more than one 
situation, as on land, and in fresh and in salt water, by Auricula (including Cono- 
vulus and Chilina)\ (2nd) of one or more species of a genus most of whose species 
inhabit fresh water being found in salt or brackish water, by Limncea, Neritina, 
Melania , and Melanopsis ; (3rd) of one or more species of a genus whose species 
usually inhabit the sea being found in fresh or brackish water, by— 
Aplysia, 
Cerithium, 
Bulla , 
Littorina ( Lithoglyphus), 
Solen ( Novaculina ), 
Tellina, 
Avicula, 
Alya, 
Corbula, 
Ostrea ( ?), 
Cucullcea {Scapula), 
Neritina ( Theodoxus ), 
Ampullaria (?), and 
Cardium. 
M Beudant found by experiment (1803—1816), that many freshwater molluscs 
can be made by degrees to live in water gradually salted to the ordinary saltness of 
the sea; and that many marine molluscs can also, by gradually diminishing the 
saltness of the water, be accustomed to live in fresh water. See ‘ Comptes Rendus,’ 
May 13th, 1816; ‘ Annales des Mines,’ 1816, vol. i. p. 397, and De la Beche’s 
‘ Selection of Geological Memoirs,’ 1824, p. 36. 
