100 MOLLUSCA FROM THE CRAG. 



Spec. Char. Testa ovatd, transversa, elongatd, crassd, valde incequilaterd ; antice 

 rotundatd, postice productd, cuneatd, subrostratd ; umbonibus rugosis. 



Shell ovate, transversely elongate, thick, very inequilateral ; anterior side rounded, 

 posterior produced, somewhat pointed, and obtusely angulated or wedge-shaped; 

 umbones rugose. 



Length, 2>\ inches. 



Locality. Stutton, Grays. Recent, Britain, France, and Germany. 



This species is very abundant at Grays, where specimens have been obtained in 

 great perfection, and although by no means rare at Stutton, they are in a very 

 decorticated condition. Both these localities present us with forms deviating con- 

 siderably from what are generally met with in the living state, more especially those 

 from the latter locality, where they attain a magnitude of nearly four inches in length, 

 and appear to have a greater proportion of the shell on the anterior side of the umbo, 

 while in those from Grays, which are smaller, that side is shorter and proportionally 

 broader than in the living specimens ; in the Grays fossil the posterior side is obtusely 

 pointed, and the whole shell is more angular, while the Stutton specimens are rather 

 less so than the general or common form of the recent shell ; as these extremes of 

 variation can readily be connected through the living species, it is presumed that the 

 differences are wholly insufficient for specific separation, and I have no hesitation in 

 assigning the fossils of both localities as identities of the existing British species; the 

 dental characters are also similar, the anterior tooth of the right valve being coarsely 

 crenated on the upper or dorsal side, and somewhat compressed ; the elongated 

 lamina on the posterior side is linear, sharp, and nearly smooth. 



It was at first thought, that as the Land and Fresh-water shells found in the 

 newer Tertiaries of this country are all assumed to be the Homogenitors of exist- 

 ing animals, a name alone with reference to a work in which they have been 

 described would have been sufficient for Geological purposes ; but upon more minute 

 examination many of them have been found to present characters deviating in so great 

 a degree, that their identity has by some Conchologists been called in question ; it is 

 therefore now considered desirable that a figure and description of a part of them at 

 least should be given, more especially as they have never yet appeared in any publi- 

 cation as British Fossils. 



3. Unio pictorum. Linnaus. 



Mya pictorum. Linn. Syst. Nat., ed. 12, No. 28, p. 1112, 1767. 



— — Poll. Test. Sicil., vol. i, p. 2, t. 9, figs. 6, 7, 1791. 

 Unio pictorum. Drap. Moll. Tert. et Fluv. de Fr., pi. 11, fig. 4, 1805. 



— Gray. Man. Land and F.-W. Shells, p. 295, pi. 2, fig. 11, 1844. 



— — Rossm. Icon. Land und Sussw. Moll., figs. 71, 196; pi. 29, fig. 409 ; 



pi. 58, figs. 762—766, 1844. 



— — Forb. and Hani. Hist. Brit. Shells., vol. ii, p. 142, pi. 39, fig. 1, and 



pi. Q, fig. 2 (Animal), 1849. 



