BIVALVIA. 65 



have been forwarded to me by Mr. Walton and by Mr. J. F. Whiteaves ; at the Oxford- 

 shire locality, the specimens are small and usually compressed. 



CoRBULA Agatha, B'Orh. Tab. XL, figs. 28, 28 a. 



CoBBULA Agatha, B'Orb. Prodrome, i, p. 307, No. 100. 



Testa parva, suhglohosa, IcBvigata, nitida, umbonibus magnis, obtusis,medianis,erectis, 

 latere anteriore rotundo ; lunula magna, concava, cordata ; latere posteriore brevi, sub- 

 carinattti tnmcata. 



Shell small, globular, smooth, shining ; umbones large, obtuse, mesial, erect ; anterior 

 side rounded, lunule large, cordiform, concave ; posterior side very short, with a faintly 

 marked oblique carina, and a truncated posterior border ; the surface has a few delicate, 

 irregular folds of growth ; it appears to be equivalve. 



The diameter through both the valves is equal to the height, and somewhat less than 

 the length. 



Corbula DesJiagsea, Buv., is also a smooth species, but less short, the posterior border 

 being also slightly sinuated. Corbula Mameillii, Mor., another smooth shell, is much 

 more oblique, and more produced posteriorly. Corbula obscura, Sow., appears to be less 

 convex, and to have the posterior side more produced. 



Geological Position and Locality. The Forest Marble of Cirencester and of 

 Wiltshire. 



Genus — Sowerbya, D' Orb., 1850. 

 IsoDONTA, Buv. Bull. Soc. Ge'ol. de Fr., ser. 2, t. 8, p. 353, 1851. 



Shell equivalve, subequilateral, the valves close fitting ; hinge in the right valve, with 

 two oblique, diverging, symmetrical cardinal teeth separated by a mesial trigonal pit, and 

 two lamellar lateral teeth separated from the hinge-border by longitudinal grooves. The 

 left valve with a projecting conical tooth between two obhque pits ; lateral teeth two, 

 longitudinal, lamellar, projecting and united to the superior border. Ligament external. 

 Muscular impressions small, rounded, deeply marked; palleal impression emarginated 

 posteriorly. 



M. D'Orbigny in his 'Prodrome de Paleontologie,' vol. i, 13 Et., p. 362, characterised 

 his genus Sowerbya as follows : — " Sowerbga, D'Orb,, 1847. — Coquille voisine des Mactra 

 par son sinus, mais avec des dents laterales enormes, et une fossette interne ligamentaire 

 simplement creusee." 



9 



