CARDITA. 125 



Description. — Shell more or less sub-quadrate, rounded, moderately convex, 

 with the postero-dorsal portion compressed ; length greater than height ; moderately 

 inequilateral. Postero-dorsal margin only slightly curved ; posterior margin more 

 or less truncated, passing by a regular curve into the ventral margin, which is only 

 slightly convex and nearly parallel with the postero-dorsal margin. Anterior 

 margin rounded. Antero-dorsal margin concave. Umbones curved anteriorly. 

 Lunule ovate, nearly smooth. Escutcheon lanceolate, with a sharp edge. 



Ornamentation consists of 47 to 57 rounded radial ribs, which are separated 

 by furrows of greater breadth than themselves ; near the postero-dorsal margin 

 these ribs are rather closer together than elsewhere. In some cases the postero- 

 dorsal part of the valve is divided into two parts by two ribs, which are more 

 prominent than the others. At regular intervals concentric lamellae occur and form 

 marked projections where they cross the ribs. Near the umbo these lamellae are 

 more widely separated, and near the margin, especially in large specimens, they 

 are closer together than elsewhere. Sometimes faintly marked concentric ridges 

 may be seen on the ribs between the lamellae. Margins of valves crenulate. 



Measurements : 



(1) (2) (3) (4) (5) (6) (7) (8) (9) (10) (11) (12) (13) (14) 



Length. 30 27 25 24 24 23-5 23 23 22 22 20 16 15 15 mm. 

 Height. 26-5 23 22 22 20 21 21 19-5 20 10 17 14-5 14 13 „ 



(1—14) Gault, Folkestone. 



Affinities. — This species has more numerous ribs and is less inflated than 

 C. neocomiensls, d'Orbigny and G. Dupiniana, d'Orbigny. 1 It possesses about the 

 same number of ribs as G. Gonstanti, d'Orbigny, 3 but is less elongate and less 

 inflated. G. rotundata, Pictet and Roux, 3 differs from G. tenuicosta in being 

 more inflated, and can be regarded as only an individual variation. Some examples 

 found at Folkestone, which in other respects agree with G. tenuicosta, are as much 

 inflated as the type of C. rotundata. G. clathrata, Buvignier, 4 is a small form, but 

 has the concentric laminae more widely separated than in even the young of 

 G. tenuicosta. G. argonnensis, Buvignier, 5 is more compressed and has more slender 

 ribs than G. tenuicosta. G. cenomanensis, d'Orbigny, is distinguished from 

 C. tenuicosta by its broader ribs and more closely placed concentric lamellae. 



G. tenuicosta has narrower ribs and broader furrows than the forms from the Chalk 



1 For references to figures of these species see p. 123, footnotes 1, 2. 



2 ' Pal. Franc. Terr. Cret.,' vol. iii (1844), p. 89, pi. cclxix, figs. 1—5 ; Pictet and Campiclie, ' Foss 

 Terr. Oct. Ste. Croix' ('Mater. Pal. Suisse,' ser. 4, 1866), p. 337, pi. cxxvi, fig. 10. 



15 ' Moll. Foss. G-res verts de Geneve' (1852), p. 443, pi. xxxiii, fig. 6. 



1 ' Statist, geol., etc., de la Meuse,' Atlas (1852), p. 19, pi. xv, figs. 16, 17. 



5 Ibid., p. 19, pi. xxxii, figs. 1—3. 



6 D'Orbigny, op. cit., p. 94, pi. cclxxxiii bis., figs. 1—4. 



