248 CRETACEOUS LAMELLIBRANCHIA. 
Description.—Shell large, elongate, somewhat arched, convex, but with the sides 
of the posterior part more or less flattened, very inequilateral, with a large gape 
at the posterior end. Anterior part short, with a rounded margin. Ventral margin 
curved. Posterior margin subtruncate, rounded. Postero-dorsal margin long, 
slightly concave or almost straight. Umbones broad. Near the umbones the 
antero-dorsal and postero-dorsal marginal parts of the shell are depressed. 
Ornamentation consists of numerous narrow, sharp, prominent and sometimes 
slightly serrate radial ribs, which are straight or shghtly curved. Near the 
anterior and the postero-dorsal margins ribs are absent; on the postero-dorsal part 
of the shell they are rather more widely separated and rather more prominent 
than on the median part. Between the ribs are broad, concave furrows which are 
crossed by numerous growth-lines. 
Measurements : 
(1) (2) 
Length b ; 126 ; WAP) aanaay, 
Height ‘ : 63 OU Ss 
(1, 2) Crackers, Atherfield. 
Affinities —The elongate form and numerous sharp ribs distinguish this from 
other Cretaceous species of Pholadomya. 
leemarks.—The foreign examples of this species show considerable variation in 
leneth, in the curvature of the shell, and in the number of ribs. Some of the 
varieties have been described as distinct species, but Pictet and Campiche have 
shown that there are numerous transitions between the different varieties. The 
English examples agree in most cases with the type of the species, but some 
approach the variety described by Agassiz as P. Scheuchzeri. A specimen figured 
by Moesch shows the large palhal sinus. 
Type.—The type came from the Hythe Beds of Court-at-Street, but cannot now 
be found. 
Distribution.—Lower Greensand (Crackers) of Atherfield. Atherfield Beds of 
East Shalford. Hythe Beds of Court-at-Street and Lympne. 
PHOLADOMYA SPEE'LONENSIS, sp. nov. Plate XLI, fig. 4a, b. 
Description.—Shell large, oval, short, inflated, very inequilateral. Anterior 
margin rounded; ventral margin convex. Umbones broad, incurved. 
Ornamentation consists of numerous (usually about twenty-one) strong, radial 
ribs, separated by broad, slightly concave interspaces. Secondary ribs are intro- 
duced at a short distance from the umbo, and usually soon become as large as the 
primary ribs. The ribs are more or less nodular. Concentric growth-lines are 
present. ‘Two or three of the anterior ribs are more widely separated than the 
others. On the postero-dorsal and the anterior parts of the shell ribs are absent. 
