﻿PROSOBRANCHIATA. 



343 



is not purely salt, and where also they are found in association with such truly freshwater 

 genera as Limncea and Planorbis. The most general distinction among existing species 

 is that the Neritina are generally smooth shells and free from spiral ridges, but this rule 

 is not without exception, as some few species possess spiral striae 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 globulus, Ferussac. Tab. XXXIV, fig. 18 a, b. 



Neritina globulus, Ferussac. Hist, des Moll. PI. de Neritines Foss., fig. 14, 1851. 



— — Desk. Coq. foss. des Env. de Par., p. 151, pi. xvii, figs. 19 20, 



1824. 



— — Desk. An. sans Vert, du Bas. de Par., t. xi, p. 22, 1858. 



— — Whitaker. Mem. Geol. Surv., vol. iv, p. 576, 1872. 



— uniplicata, J. Sow. Min. Conch., t. ccclxxxv, figs. 9, 10, 1823. 



— callifera ? G. B. Sow. Genera of shells ; Neritina, fig. 7, 1855. 



Spec. Char. N. Testa ovato-globulosd, spird brevissimd,planiusculd, anfractibus tribus, 

 ultimo maximo oblongo Icevigato, supra convexo, subtus concavo ; aperturd magna late 

 semilunari ; area columellari plana, laevigata, margine acuto, in medio paulo excavato, 

 posterius unidentato. 



Diameter, \ of an inch. 



Localities. Brit. : Charlton (S. Wood), Plumstead, Peckham, New Cross {Meyer). 

 Prance: Epernay, Mont Bernon (Des/iayes). 



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 Prance. 



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 striae on the exterior. This is a plain-looking shell ; 

 and sometimes portions of the epidermis have been preserved on the specimens. 



