254 CRETACEOUS LAMELLIBRANCHIA. 



the figures given by several authors, that species varies considerably, so that it 

 seems probable that when more English specimens have been obtained it will be 

 possible to refer them definitely to M. semicostata. 



Distribution. — Lower Greensand of Furze Hill, Faringdon. 



Genus — Gon'iomya, L. Ar/assi;:, 1842. 

 (' Ktudes t-rit. Moll. Foss.," Mves, pp. xii, 1.) 



GoNioMYA Archiaci {Pictct and Eenevlrr), 1855. Plate XLII, figs. 4, 5. 



1855-6. Thracia Archiaci, F. J. Pidet and E. Beneiner. Foss. Terr. Aptieii 



(Matt'r. Pa,l. Suisse, ser. 1), p. 67, 

 pi. vii, fii,'. 5. 

 1858. - — Pidet and Renevier. Ibid., p. 176. 



1865. Pholadomya thracioides, F. J. Pidet and G. Campidie. Terr. Crut. Ste. 



Croix (Mater. Pal. Suisse, ser. 

 4), p. 92 (Gohiomi/a). 

 1870. GoNioMYA Archiaci, F. Stoliczka. Palseout. Iiidica, Cret. Fauua S. liidia, 



vol. iii, p. 75. 



Description. — Shell ol^long, convex, nearl}^ equilateral. Anterior margin 

 rounded. Ventral margin slightly convex or nearly straight, ncai'ly parallel with 

 the dorsal margin. Posterior margin truncated, oblique, slightly convex, forming 

 a rounded angle with the ventral margin and an obtuse angle with the postero- 

 dorsal margin. Umbones of moderate size, with a faint rounded carina extending 

 postero-ventrally, above which the shell is compressed ; lunule and escutcheon 

 elongate, depressed, limited by carina?. 



Ornamentation : Anteriorly to the umbones there is a ribbed area in which the 



ribs extend oblicjuely backwards ; posteriori}' to the umbones is another ribbed area 



in which the ribs are nearly perpendicular to the postero-dorsal margin, Ijut 



slightly curved ventrally. The anterior, ventral, and postero-dorsal parts of the 



shell are Avithout ribs, and show growth-lines only. Near the umbo the ril)s of 



the two areas are connected by horizontal ribs, but ventrally these horizontal ril)s 



are absent or indistinct. Some parts of the shell are marked by rows of small 



pits. 



Measurements : 



(1) (2) (3) 



Length . . 33 . 28 . 19 mm. 



Height . . 19 . IG . 11 „ 



'& 



(1 — 3) Crackers, Atherfield. 



