46 CRETACEOUS LAMELL1BRANCHIA. 



Noil 1837. Arca cabinata, A. Goldfuss. Petref. Germ., vol. ii, p. 150, pi. cxxiv, 



fig. 2. 

 — 1837. — — F. C. L. Koch and W. Bunker. Norddeutsch. Oolithgeb., 



p. 32, pi. ii, fig. 14. 



Description. — Shell elongate, trapezoidal, inflated, inequilateral, rather short 

 anteriorly. Dorsal and ventral margins nearly parallel. Anterior margin joining 

 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 stria?. 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, curviug obliquely 

 outwards from under the umbo; anterolaterals oblique; postero-laterals long 

 and parallel to the hinge-line. Posterior adductor impression without projecting 

 edge. Margins faintly crenulate. 1 



Measurements : 



(1) (2) (3) (4) (5) (G) 



Length 29 45 22 21 24"5 30 mm. 



Height 10 29 12 135 17 21 „ 



Thickness . . . . — — — 12 15 21*5 „ 



(1) is from the Upper Greensand of Devizes ; (2) and (3) are from the Gault of Folkestone ; 

 (4 — 6) are from the Blaekdown Greeusand. 



Affiiiities. — This species is closely related to Cucullsea 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 Cucullsea costellata, Sowerby, from the Blackdown 



1 Seen only in n few well-preserved bpecimens. 



