256 CRETACEOUS LAMELLIBRAXCHIA. 



Ornamentation consists of rounded riljs ; those in front of tlic niabones are 

 nearly straight and slope obliquely backwards and downwards ; those behind the 

 umbones are curved and more or less nearly parallel to the posterior and postero- 

 dorsal margins; near the ventral margin the ribs become more nearly concentric 

 and are often less distinct (especially on the median part) than on tiie dorsal 

 {lortion of the shell. 



AQinitiea. — This species differs from (j. ih'.-^iijnuta (Goldfuss)' in the smaller 

 curvature of the ventral margin, and in the truncated form of the posterior 

 margin. It is closely allied to (I . ciiusiijuiitu, Romer,- but the anterior curvature 

 of the umbones is more marked. 



Remarks. — This species has been identified b}' comparison with a specimen 

 given me by M. Raoul Fortin from the same locality as the type. 



Ttjpe. — From the Cenomanian of Mte. Ste. Catherine, Rouen. 



Disfrihnfion. — Upper Greensand (zone of Bfltlifnhachia roatrnta) of Blackdown, 

 Devizes, and near Maiden Braillev. Base of Chalk Marl of Chard. ^ 



i^(u;i/Z//— PLEUROMYID.^, Z///e/. 



Genus — P;.EDR0MTA, L. Aijassiz, 1842. 

 (' Etudes crit. Moll. Foss.,' Myes, p. 231.) 



Pleuromya Orbignian.v (Uoiiilliev), 1847. Plate XliTTI, figs. 1, 2 a — c. 



1847. PanoPjEa. Obbigniana, C. Rouillier. Bull. Soc. Imp. Nat. Moscou, vol. xx, 



p. 407; vol. xxi (184.8), p. 281, 

 pi. G, fig. 24. 



JJescrijjf.ion. — Shell oval, convex, inequilateral, with a small posterior gape. 

 Anterior and posterior margins rounded. Ventral margin considerably convex. 

 Umbones moderately prominent, incurved. Shell depressed in front of and l)ehind 

 the umbones. Surface nearly smooth, but ornamented with linear, radial ribs, 

 which are sometimes parallel, sometimes irregular; in some places the ribs are 

 replaced by rows of minute tubercles. 



Affinities. — The English specimens agree closely with examples from the Lower 



1 ' Petref. Genu.,' vol. ii (1840), p. 264, pi. cliv, fig. 13. Holzapfel, ' Mollusk. Aachen. Kreide ' 

 (Palseoutographica, vol. xxxv, 1889), p. 153. 



■ 'Die Versteiu. d. nord-deutsfli. Kreidegeb.' (1841), p. 75, pi. x, fig. 3. Midler, 'Mollusk. 

 Untersen. v. Braunschweig u. Ilsede' (1898), p. 71, pi. x, fig. 7. 



^ A small spcL'iniou of Goniunn/a has beeu found in the Chalk of Triiuiugham by Mr. R. M. 

 Brydone. 



