8 FOSSIL ESTHERLE. 



species, the following are the most prominent : — Rlssoa (Assiminia), Cerithium (Potamides), 

 Area (Sca/phda), Solecurtus (Novaculina), Mytilus (Dreissena), and Cardium, 1 but how the 

 extinct genera were circumstanced in this respect, and whether the old species of extant 

 genera had similar habitats to those of their existing congeners, can only be partially 

 surmized, chiefly from the evidence of the best known of their associates. 



There are some existing genera the species of which appear to be essentially fluviatile, 

 but live also in company with true marine shells in the mouths of rivers ; these are Cyrena 

 and Awpullaria. Such, too, may have been the habit of the old Estherice ; at all events, 

 there is no necessity for supposing them to have been marine ; but where they occur by 

 themselves, or in the company only of fishes 2 and plants, 3 they may be regarded as having 

 lived and died in fresh (or possibly brackish) water ; where they are mixed with shells of 

 presumed marine character, they indicate probably that fresh water was in close proximity to 

 the place of deposit, if it had not been replaced by the sea by possibly frequent alternations. 



We must not forget, however, that, judging by analogy, the Entomostracous Crusta- 

 ceans under notice may have been capable of living, at least for considerable periods, in 

 even salt water, for some of the common Cyprides, such as are abundant in freshwater 

 streams, are not uncommon in ditches of brackish and even highly saline water in the low 

 grounds near the sea. 



1 In Dr. J. E. Gray's " Memoir on Testaceous Mulluscs," in the ' Pbilos. Transact.' for 1835, he treats of 

 " Species of Testaceous Mollusca living in very different situations from the majority of the known species 

 of the genus to which they belong, or having the faculty of maintaining their existence in several different 

 situations;" and 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 Conovulus 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, 



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 freshwater. 

 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. 



2 With some exceptions, it is impossible to say of any fossil fish that it did, or that it did not, belong 

 exclusively to the sea, even when it is occasionally associated with marine fossils, as some of tbe Old Red 

 fishes are in Russia. Many genera of fishes are as capricious, as to the habitats of their species, as the above- 

 quoted molluscs are. Nor must we forget that the stony-scaled and plated fishes of Palaeozoic times are now 

 best represented by the Bicbirs and the Sheat-fishes of existing rivers (Huxley ; ' Mem. Geol. Surv.,' 1861). 



3 The association of remains of land-plants with the Estherice is of frequent occurrence. 



Mya, 



Corbula, 



Ostrea (?) 



Cucullcea (Scapula), 



Neritina (Theodoxus), 



Ampullaria (?), and 



Cardium. 



