336 CRETACEOUS LAMELLIBRANCHIA. 



Inoceeamus costellatus, sp. nov. Plate LIV, figs. 5 — 7. 



1897. Inoceeamus, sp., H. Woods. Quart. Journ. Geol. Soc, vol. liii, p. 381, 



pi. xxvii, figs. 14 — 17. 



Description. — Shell small, very inequilateral, rather higher than long, of 

 moderate convexity — the greatest convexity being between the umbones and the 

 postero-ventral extremity. Umbones terminal. Hinge equal to about three- 

 quarters of the length of the shell, and forming more than a right angle with the 

 anterior margin. Anterior margin moderately convex, ventral margin very 

 convex, posterior margin slightly convex. 



Right valve with a small, pointed, slightly curved umbo. Posterior and postero- 

 dorsal parts of the valve flattened A small, flattened antero-dorsal area is nearly 

 perpendicular to the plane of the valves. 



Left valve more convex than the right ; postero-dorsal part compressed, but 

 not forming a definite ear. Umbo narrow, pointed, curved inwards, larger and 

 more prominent than the umbo of the right valve. Antero-dorsal area larger 

 than on the right valve. 



Concentric ribs narrow, sharp, usually widely separated; interspaces broad 

 and shallow. The curvature of the ribs is very unsynmietrical ; the ventral part 

 is very convex, the posterior part only slightly convex. 



Remarks. — This species is fairly common in the Chalk Rock. All the speci- 

 mens seen are casts. Some examples, Avhich appear to be a variety of this species, 

 have small ribs of uniform size. 



Affinities. — This species appears to be related to some forms of I. LamarcH, 

 Parkinson, but the left umbo is narrower, more pointed, and less curved ; the line 

 of greatest convexity is more oblique to the hinge-line, and the posterior and 

 postero-dorsal parts of the valves are more compressed. 1 



Type. — From the Chalk Rock of Cuckhamsley, in the Sedgwick Museum, 

 Cambridge. 



Distribution. — Chalk Rock of Dover, Guilford Colliery (Coldred near Dover), 

 the Sussex Coast, south-east of Calstone TVillington, Cuckhamsley, Blount's Farm 

 near Mario w, Luton, Wallington near Baldock, Barley near Royston, and 

 Underwood Hall near Dullingham. Zone of Holaster planus of South Devon, the 

 Dorset Coast, the Isle of Wight, Lichfield (Hants), Winchester, etc. 2 



1 Compare also I. undulatus, Eogala, 'Bull. Iuternat. Acad. Sci. Cracovie' (1911), p. 171, pi. iv, 

 fig. 7, and I. Frechi, Audert, ' Inocerameu d. Kreibitz-Zittauer Sand stein geb.' (1911), p. 51, pi. i, 

 fig. 8. 



2 Recorded by Rowe from the zones of Terebralulina lata and Micraster cor-anguinum of Dover. 



