PROSOBRANCHIATA. 343 
is not purely salt, and where also they are found in association with such truly freshwater 
genera as Limnea and Planorbis. The most general distinction among existing species 
is that the Weriting are generally smooth shells and free from spiral ridges, but this rule 
is not without exception, as some few species possess spiral strize or depressed ridges. 
The form of the species of this genus varies, but in a slight degree, and the larger 
number of the fossils have the external markings well preserved ; but these markings are 
so variable on specimens even of the same species, not unfrequently resembling the mark- 
ings usual on other species, that they do not afford much assistance in their specific 
separation. The dark lines seem to be as well preserved on some of our fossils as are the 
red spots on other shells of older date; so that the preservation of colour on several of 
our Tertiary species is not reserved to red alone. 
No. 264. Neritina cLosuius, Férussac. Tab. XXXIV, fig. 18 a, 6. 
NERITINA GLOBULUS, Férussac. Hist. des Moll. Pl. de Neritines Foss., fig. 14, 1851. 
— _ Desh. Coq. foss. des Env. de Par., p. 151, pl. xvii, figs. 19 20, 
1824, 
== == Desh. An. sans Vert. du Bas. de Par., t. xi, p. 22, 1858. 
— — Whitaker. Mem. Geol. Surv., vol. iv, p. 576, 1872. 
— unrpLicaTa, J. Sow. Min. Conch., t. ecclxxxv, figs. 9, 10, 1823. 
a CALLIFERA ? G. B. Sow. Genera of shells; Neritina, fig. 7, 1855. 
Spec. Char. N. Testa ovato-globulosd, spira brevissimd, planiusculd, anfractibus tribus, 
ultimo maximo oblongo levigato, supra convexo, subtus concavo ; apertura magna late 
semilunart ; ared columellari pland, levigatd, margine acuto, in medio paulo eacavato, 
posterius unidentato. 
Diameter, + of an inch. 
Localities. Brit.: Charlton (S. Wood), Plumstead, Peckham, New Cross ((/ever). 
France: Epernay, Mont Bernon (Deshayes). 
This is by no means rare, and it appears to be restricted to the Lower Eocene Forma- 
tion in England, as it is also said by M. Deshayes to be in France. 
The apex of this shell is very slightly elevated, and generally more or less eroded ; 
the volutions are indicated by a narrow line of suture ; the columella is broad, flat, and 
moderately sharp when the specimen is in good condition, and furnished with one 
prominent tooth at the upper part, but on the lower the denticles are obsolete or very 
minute, and there are no spiral striz on the exterior. ‘his is a plain-looking shell ; 
and sometimes portions of the epidermis have been preserved on the specimens. 
