24 CRETACEOUS LAMELLIBRANCHIA. 
less distinct carina. When, however, a considerable number of specimens are 
compared it is difficult to draw any line of separation between these forms and 1. 
Sancte-Crucis. But since, in most cases, the original shape of the shell has been 
more or less considerably modified by pressure, it is not easy to come to a definite 
conclusion in this matter; moreover, in both 17’. Sancte#-Crucis and the larger forms 
there is clearly some variation in relative height and length of the shells, and in 
the distinctness of the carina.! 
Distribution —Gault of Black Ven and Folkestone. Upper Greensand of 
Blackdown and Devizes. 
THRACIA CARINIFERA (Sowerby), 1826. Plate XL, figs. 10—13. 
y 8 
1826. Lurraria? caRrInireRA, J. de C. Sowerby. Min. Conch., vol. vi, p. 66, pl. 
dxxxiv, fig. 2. 
1832. — — A. Passy.  Descript. géol. de la Seine-Infér., 
Atlas, p. 6, pl. xiii, figs. 6, 7. 
1842. Corrmya carinirera, L. Agassiz. Etudes crit. Moll. Foss., Myes, p. 264. 
1845. Lyonsta cARINIFERA, A. d’Orbigny. Pal. Franc. Terr. Crét., vol. iii, p. 385, 
pl. ccelxxiii, figs. 1, 2. 
1850. — --- A. d Orbigny. Prodr. de Pal., vol. ii, p. 158. 
1852. —- — R. Kner. Denkschr. d. k. Akad. d. Wissensch. Wien, 
Math.-nat. Cl., vol. i, p. 311. 
1854. Lurrarta CARINIFERA, J. Morris. Cat. Brit. Foss., ed. 2, p. 208 (? Thracia). 
1865. THracta carinirera, F’. J. Pictet and G. Campiche. Foss. Terr. Cret. Ste. 
Croix (Matér. Pal. Suisse, ser. 4), p. 121. 
1870. — — F.. Stoliczka. Palezeont. Indica, Cret. Fauna 8. India, 
vol. ii, p. 72. 
1893. Lyonsta CARINIFERA, A. Fritsch. Stud.im Gebiete der bohm. Kreideformat. 
v. Priesen. Schicht., p. 97, fig. 115. 
Description.—Shell thin, oval, of moderate convexity, slightly mequivalve and 
inequilateral. Anterior margin rounded, passing gradually into the regularly 
convex ventral margin. Posterior margin truncated, straight or slightly concave. 
Postero-dorsal margin nearly straight. Umbones broad, incurved close together, 
with a sharp carina passing to the postero-ventral angle and limiting a flattened 
or slightly concave postero-dorsal area, which is divided in the middle by a shallow, 
longitudinal furrow. Just in front of the carima is a broad, shallow, concave 
depression. 
Ornamentation consists of broad, slightly raised, concentric folds which are 
more distinct on the anterior part than elsewhere. On the postero-dorsal area are 
1 A similar case of distortion and variation is furnished by Thracia semiplanata, Whiteaves, 
‘ Mesozoic Foss.’ (‘ Geol. Surv. Canada’), vol. i (1884), p. 221, pl. xxix, fig. 5. 
