248 CRETACEOUS LAMELLIBRANCHIA. 



Description. — Sliell large, eloniifate, some what arched, convex, l)ut with tlie sides 

 of the posterior part more or less flattened, very ine(|uilateral, with a large gape 

 at the posterior end. Anterior part short, with a rounded margin. Venti'al margin 

 curved. Posterior margin sul)truncate, roiuukcl. Po.stero-dorsal margin long, 

 slightly concave or almost straight. Umbones broad. Near the nnd)ones the 

 antero-dorsal and postero-dorsal marginal parts of the shell are depressed. 



Ornamentation consists of numerous narrow, sharp, prominent and sometimes 

 slightlj- serrate radial ribs, which are straight or slightly curved. Near the 

 anterior and the postero-dorsal mai-gins ribs are absent ; on the postero-dorsal part 

 of the shell they are rather inore widely separated and i-ather more jn-ominent 

 than on the median ])art. Between the ribs are broad, concave fun-ows wliicli aie 

 crossed by numerous growth-lines. 

 Measiivfments .• 



(1) (2) 



Length . . 12() . 122 mm. 



Height . . 63 . GO „ 



(1, 2) Crackers, Atlierfiekl. 



Jljiiiitirs. — The elongate form and numerous sharp ribs distinguisli tliis from 

 other Cretaceous species of Plioladomi/a. 



lii'niarls. — The foreign examples of this species show considerable vaiiatii)ii in 

 length, in the curvature of the shell, and in the nuiuber 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 

 ajjproach the variety described by Agassiz as P. Scheuchzeri. A specimen figured 

 by Moesch shows the large pallial sinus. 



Ti/pe. — The type came from the HytheBedsof Court-at-Street, but cannot now 

 be found. 



Distfibufioii. — Lower Greensand (Crackers) of Atherfield. Atherfield Beds of 

 East Shalford. Hythe Beds of Court-at-Street and Lympne. 



Puor.AiioMVA si'KKTONENSis, sp. uov. Plate XLT, fig. L', A. 



Description. — Shell large, oval, short, intlated, very inecpiilateral. Anterior 

 margin rounded; ventral margin convex. Umljones broad, incurved. 



Ornamentation consists of numerous (usually about twenty-one) strong, radial 

 ribs, separated by broad, slightly concave interspaces. Secondary ribs are iiitru- 

 duccd at a short distance from the umbo, and usually soon become as large as the 

 ])riniary 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. 



