428 CRETACEOUS LAMELLIBRANCHIA. 



Remarks. — The type cannot be found, but other specimens named by Forbes 

 are in the Museum of Practical Geology (Geological Society Collection, No. 2217). 

 Distribution. — Lower Greensand (Crackers) of Atherfield. 



Unicardium? compressum, sp. uov. Plate LXII, figs. 5, 6. 



[Omitted from Vol. II, p. 163.] 



Description. — Shell convex, with flattened or slightly concave flanks, oval or 

 subquadrate, moderately or slightly inequilateral. Anterior margin rounded ; 

 posterior margin subtruncate, slightly oblique, curved ; ventral margin nearly 

 straight and nearly parallel to the dorsal margin. Umbones broad, close 

 together. A rounded carina extends from the umbo to the postero-ventral 

 extremity, limiting the compressed postero-dorsal region. External ligament broad. 

 Ornamentation consists of a few coarse somewhat irregular concentric ribs, which 

 are sometimes rugose, with finer ribs in the interspaces. 

 Measurements : 



(i) (2) 



Length 40 . 32 mm. 



Height 26 . 22 „ 



Thickness . . . . . . .17 . — ,, 



(1, 2) Crackers, Atherfield. 



Affinities. — The hinge and interior of this species have not been seen, conse- 

 quently its generic position cannot at present be definitely determined, but it is 

 provisionally referred to Unicardium on account of its external resemblance to 

 some Jurassic species of that genus. U.? compressum is similar to some forms of 

 Mya rugosa, Romer, 1 which lias been referred to the genus Lueina by de Loriol 

 and Cotteau, to Mactromya by Agassiz, and to Unicardium by Zittel. 



Type. — From Atherfield, in the Sedgwick Museum, Cambridge. 



Distribution. — Lower Greensand (Crackers), of Atherfield. Atherfield Clay of 

 Dover Colliery. 



Unicardium ? Mailluanum (d'Orbigny), 1844. Plate LXII, figs. 7—9. 



[Omitted from Vol. II, p. 165]. 



1844. Cardium Mailleanum, A. d'Orbigny. Pal. Franc. Terr. Crct., vol. iii, p. 



40, pi. cclvi, figs. 7—12. 



1 See especially de Loriol and Cotteau, 'Mod. Palcont. Gcol. Portlandieu de l'Yonne ' (1868), 

 p. 135, pi. ix, figs. 10, 11. 



