34. CRETACEOUS LAMELLIBRANCHIA. 
Affinities.—See A, Sancte-Crucis (below). The external teeth are more oblique 
in this than in most forms of Avea, in which respect it may be compared with 
A. equidens, Tate.’ 
Distribution.—Lower Greensand of Upware. Atherfield Beds of East Shalford 
and Peasmarsh. ? Perna-bed of Redcliff. 
Arca Sanora-Cructs, Pictet and Campiche, 1866. Plate VI, figs. 6 a—c, 7 a—e. 
1866. Arca Sancrz-Cructs, #. J. Pictet and G. Campiche. Foss. du Terr. Cret. 
de Ste. Croix (Matér. Pal. Suisse, ser. 4), pt. 3, 
p. 487, pl. exxx, figs. 10, 11. 
Description.—Shell elongate, very convex in the umbonal region; wedge-like 
posteriorly. Anterior margin rounded, more or less perpendicular to the hinge- 
line. Ventral margin a little sinuous, more or less parallel to the dorsal. Poste- 
rior margin nearly straight, and either at right angles or slightly oblique to the 
hinge-line. Umbones prominent, incurved, placed near the anterior third of the 
shell. Carina rounded; postero-dorsal area depressed, sometimes slightly con- 
cave, with one or two faint broad ridges. Hinge-area long and broad. Orna- 
mentation consists of numerous fine radial ribs crossed by lines of growth. 
Interior not seen. 
Measurements : 
Q) (2) 
Length . : : ool : . 205 mm. 
Height . : : eli, , 2) LA 
Thickness : : Gro 15 
» 
(1) aud (2) are from Upware. 
Affinities.—This form agrees in many respects with A. Carteroni, d’Orbigny, 
and it is hkely that a large series of specimens would show a passage between the 
two. At present I have seen only three or four examples of each. Arca 
Sancte-Crucis appears to differ from A. Carteroni in being proportionately 
shorter, in having the ventral margin sinuous and less parallel to the dorsal, and 
in the umbones being more prominent and incurved. 
It also appears to be similar to Arca autissiodorensis, Cotteau,’ but seems to be 
less inflated and to have the posterior margin less oblique. 
1 «Trans. Roy. Soc. S. Australia,’ vol. viii (1886), p. 189, pl. xi, fig. 9; G. F. Harris, ‘Cat. Tert. 
Mollusca’ (Brit. Mus.), part i (1897), p. 331. 
* «Moll. Foss. de ’Yonne’ (1855), p. 86; de Loriol and Cotteau, ‘Mon, Pal. Géol. de Port- 
landien de l’Yonne’ (1868), p. 179, pl. x, figs. 6, 7. 
