46 CRETACEOUS LAMELLIBRANCHIA. 
Non 1837. Anca carinara, A. Goldfuss. Petref. Germ., vol. il, p. 150, pl. exxiv, 
fig. 2. 
-—— 18387. — — F.C. L. Koch and W. Dunker, Norddeutsch. Oolithgeb., 
p. 32, pl. ii, fig. 14. 
Description.—Shell elongate, trapezoidal, inflated, inequilateral, rather short 
anteriorly. Dorsal and ventral margins nearly parallel. Anterior margin joming 
the ventral in a gradual and regular curve, but forming with the hinge-line a 
sharp angle, which constitutes the anterior extremity of the valve. Posterior 
margin oblique, more or less sinuous ventrally, and forming sharply-marked 
obtuse and acute angles with the dorsal and ventral margins respectively. 
Umbones rather prominent, much incurved, with a sharp, finely crenulate carina, 
which extends in a double curve to the postero-ventral angle, and thus cuts off 
from the rest of the shell a deeply concave postero-dorsal area. Hinge-line nearly 
or quite equal to the length of the shell. Hinge-area fairly broad, with a varying 
number (often six or more) ligament-grooves. Surface of shell ornamented with 
numerous, distinct, rather flattened radial ribs, separated by narrower grooves, 
and sometimes crossed by concentric striz#. On the anterior part of the shell the 
radial ribs become narrower, much more elevated, more widely separated, and are 
crossed by distinct ridges; smaller ribs may be seen in the grooves. In most 
specimens well-marked growth-lines may be seen at rather distant intervals. On 
the postero-dorsal area the ribs are narrow and separated by wider depressions ; 
these ribs are more or less crenulate, and are cut by fine concentric ribs, giving a 
cancellated appearance ; a central, and sometimes also two other inner radial ribs, 
are more elevated than the others. Teeth finely striated, curving obliquely 
outwards from under the umbo; antero-laterals oblique; postero-laterals long 
and parallel to the hinge-line. Posterior adductor impression without projecting 
edge. Margins faintly crenulate.’ 
Measurements : 
GQ) (@) ©) (5) (6) 
Length . F : ; . 29 45 22 21 245 30 mm. 
Height . : : : BGS 2 90 e226 sail. ese ee 
Thickness : j F ._—- — — 12 15 PANO ecy 
(1) is from the Upper Greensand of Devizes; (2) and (3) are from the Gault of Folkestone ; 
(4—6) are from the Blackdown Greensand. 
Affinities —This species is closely related to Cucullaa securis, Leymerie (vide 
supra). 
Remarks.—The form described by Sowerby as Arca carinata came from the 
Upper Greensand of Devizes Canal; it has been regarded by d’Orbigny and some 
other authors as identical with Cucullea costellata, Sowerby, from the Blackdown 
? Seen only in a few well-preserved specimens. 
