104 MOLLUSCA FROM THE CRAG. 



from the fresh-water genus Cyclas, excepting in their shelly covering, which in this shell 

 is thick and opaque, while in Cyclas it is thin and corneous or semitransparent. Lamarck 

 placed them in his Family Conchse fluviatiles, in consequence of a resemblance to the 

 animals of the Veneridse, and from possessing the lobes of the mantle prolonged into 

 siphonal tubes, distinct and separated down to their bases. 



Although in the recent state, this is a genus of pure fresh-water origin ; specimens 

 have been found in the Estuary Deposit of the Norwich or Mammaliferous Crag almost 

 too numerous to be considered as entirely of accidental introduction. 



1. Cyrena consobrina, Caillaud. Tab. XI, fig. 15, a — c. 



Cyrena consobrina. Cail. Voy. en Egypt, t. 2, pi. 61, figs. 10, 11, 1823. 



— — Desc. de l'Egypt Hist. Nat., t. 22, p. 193, pi. 7, fig. 7, 1, 2, 3. 



— trigonula. S. Wood. Mag. Nat. Hist., vol, vii, p. 275, fig. 45, a, b, c, 1834. 



— — Lyell. Elem. Geol., 2d ed., vol. i, p. CI, fig. 26, 1841. 



— Gemmellarii. Phil. En. Moll. Sic, vol. i, p. 39, t. 4, fig. 3, 1836. 



— — Id. - vol. ii, p. 31, 1844. 



— Duchastellii. Nyst. Bull, de la Soc. de l'Acad. Roy. de Brux., p. 113, pi. 1, 



figs. 1—4, 1838. 



Spec. Char. Testa rotundato-trigonuld, subaquilaterd, tumidd, crassd; lineis elevatis, 

 concentricis, Icevibus, distantibus ; cardine tridentato, diveryens, inter se insertis ; dentibus 

 lateralibus lonyissimis, perpendiculariter striatis. 



Shell roundly trigonal, subequilateral, tumid, and thick ; externally ornamented 

 with smooth, concentric, sharp, and distant ridges ; hinge with three cardinal diverging 

 teeth in each valve, lateral teeth elongated and perpendicularly striated. 



Locality. Mam. Crag, Bramerton, Wangford, and Bulcham {Alexander). 



Stutton and Grays. Recent, River Nile. 



This species is exceedingly abundant in the purely Fresh-water Deposit at Stutton, 

 where the valves are commonly united, as they are in general with fresh-water species, 

 individual specimens may be obtained by hundreds. When the shell was first 

 described and figured in the ' Mag. Nat. Hist.,' as referred to above, it was imagined to 

 be specifically distinct in consequence of the posterior side being somewhat angulated, 

 in which character it differs from the general form of the recent species, now con- 

 sidered as identical ; but among a large series of the British fossils this character 

 disappears, and as a distinguishing mark cannot be faithfully relied upon, as the 

 specimens from Grays do not possess it, but have both sides more rounded, and 

 correspond in form precisely with the Nile shell ; there is every reason, therefore, to 

 believe its descendant to be now living in the rivers of Egypt, to which part of the 

 world it appears to have retired through the once existing fresh-waters of Sicily, for 

 I consider the shell figured by Philippi, as nothing more than a variety of this species, 

 although he has described it as having only two cardinal teeth in each valve, while 

 there are three perfectly distinct in our shell ; the anterior one in the right valve and 



