ORIN US: OB, 
Marresta ? rotunpa (Sowerby), 1850. Plate XXXVIII, fies. 12 a—e, 13. 
1850. 'TEREDO rotuNDUus, J. de C. Sowerby in F. Dixon. Geol. Sussex, p. 346 (p. 
382, ed. 2), pl. xxviii, figs. 27, 28. 
1854. — — J. Morris. Cat. Brit. Foss., ed. 2, p. 227. 
1897. Marrtesta? rotunpa, H. Woods. Quart. Journ. Geol. Soc., vol. li, 
p. 393, pl. xxvii, figs. 15—18. 
Description.—Shell small, ovoid, inflated, with rounded outline. Anterior gape 
large, apparently closed by callus. Umbones sub-median, curved inwards and 
forwards. <A strong groove and a corresponding internal rib pass from the umbo 
to the ventral margin. Near the dorsal margin a strong, narrow internal rib passes 
backward from the umbo but does not reach the posterior margin. Ornamentation 
posterior to the groove consists of concentric ribs and of two radial ribs imme- 
diately behind the groove ; anterior to the groove are a few regular radial ribs, and 
concentric ribs parallel to the margin of the anterior gape. Length, 7 mm. ; 
height, 5°5 mm.; thickness, 5 mm. 
Remarks.—I have seen only three casts of the interior and two of part of the 
exterior of this species, so that at present it is difficult to determine satisfactorily 
its generic position. 
T'ype.—The type came from the Chalk of Kent, but cannot now be found. 
Distribution.—Chalk Rock of Cuckhamsley (Berkshire), and of Whyteleafe 
(Surrey). 
Genus—Tournus, W. M. Glabb, 1864. 
(‘ Geol. Surv. California,’ Paleontology, vol. i, p. 145.) 
Turnus Datiast (Walker), 1866. Plate XXXVIII, figs. i4 a, b, 15. 
1866. Puoxas Dauuasti, J. F. Walker. Ann. Mag. Nat. Hist., ser. 3, vol. xviii, 
p. 386, pl. xiii, figs. 3, 4. 
Description.—Shell ovate, short, inflated in front, wedge-like behind. Margin 
of the anterior gape oblique, gently curved. Posterior margin rounded. Umbones 
prominent, curved inwards and forwards. A narrow furrow, and a corresponding 
internal rib, extend from the umbo to the ventral margin in a somewhat oblique 
direction, and the furrow is crossed by small ribs. A broad and strong internal rib 
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