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. 
Corsuta Acatua, D’Ord. Tab. XL, figs. 28, 28 a. 
Corsuta AGatHa, D’Orb. Prodrome, i, p. 307, No. 100. 
Testa parva, subglobosa, laevigata, nitida, umbonibus magnis, obtusis, medianis, erectis, 
latere anteriore rotundo; lunula magna, concava, cordata; latere posteriore brevi, sub- 
carinata, truncata. 
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 Deshaysea, Buv., is also a smooth species, but less short, the posterior border 
being also slightly sinuated. Corbula Macneillii, Mor., another smooth shell, is much 
more oblique, and more produced posteriorly. Cordula 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’ Oré., 1850. 
Isoponra, Buv. Bull. Soc. Géol. de Fr., sér. 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 oblique 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 Paléontologie, vol. i, 13 Et., p. 362, characterised 
his genus Sowerbya as follows :—‘ Sowerbya, D’Orb., 1847.—Coquille voisine des Mactra 
par son sinus, mais avec des dents laterales Gnormes, et une fossette interne ligamentaire 
simplement creusée.”’ 
9 
