270 
ORIGINAL ARTICLES. 
extinct genera were circumstanced in tins respect, and whether the 
old species of extant genera had similar habitats to those of their 
existing congeners, can only be partially surmised, 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 Ampullaria. Such, 
too, may have been the habit of the old Esther ice: 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* 
and plants,f 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 either that they 
inhabited brackish lakes, with a quasi-marine fauna (such as that of 
the Caspian Sea), or that fresh water was in close proximity to the place 
of deposit, if indeed it had not been replaced by the sea by, possibly, 
frequent alternations. 
We must not forget, however, that, judging by analogy, the En- 
tomostracous Crustaceans 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. 
The following Table shows the distribution of the fossil Estherice 
and Leaice; J and indicates the organic remains found with them: — 
* 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 the Old Red fishes are in Russia. Many genera of 
fishes are as capricious, as to the habitats of their species, as the above-quoted mol¬ 
luscs are. Nor must we forget that the stony-scaled and plated fishes of Palseozoic 
times are now best represented by the Bichirs and the Sheat-fishes of existing rivers 
(Huxley; ‘ Mem. Geol. Surv.,’ 1861). 
f The association of remains of land-plants with Estherice is not unfrequent. 
The occurrence of Bivalved Entomostracans with fish-remains is frequent in the 
fossil state, and agrees with the known habits of these animals. Entomostraca , like 
other Crustacea , act as scavengers among dead molluscs and fishes ; and to many 
fishes they are an important article of food. 
J Lcaia is a problematical ally of Estheria; the Cypricardia Leidyi, Lea, 
is the only form of it hitherto published. It belongs to the Carboniferous Period. 
