GYROPLEURA. 209 



of the type) and with d'Orbigny's figs. 4 and 5. The ornamentation in this 

 species is imperfectly ' known ; most of the examples from Rouen are either 

 internal casts or have only portions of the shell present, on which the orna- 

 mentation is rather indistinct. In one specimen from Dorset part of the shell 

 of the fixed valve is preserved ; the radial ribs are not so prominent as in 

 G. inequirostrata, but concentric lamellae are distinct. 



Distribution,. — Chloritic Marl of Melbury Park and Chaldon, Dorset. Recorded 

 by Jukes-Browne from the base of the Lower Chalk of Chard and Maiden 

 Newton. 



GtYROPLEura TNEQUIROSTRATA (Woodward), 1833. Plate XXXITI, figs. 8 — 13. 



1833. Diceras inequirostratus, S. Woodward. Geol. Norfolk, p. 47, pi. v, fig. 22. 

 1854. Chama inequirostrata, J. Morris. Cat. Brit. Foss., ed. 2, p. 194. 



Description. — Shell much inflated. Right valve large, oval, attached by a 

 considerable portion of the anterior side ; umbo prominent, pointed, incurved 

 anteriorly. Left valve capuliform, with the umbo near the hinge-margin. 



Ornamentation consists of strong, lamellar radial ribs, which are undulose 

 where they cross growth-rings. The ribs have strongly serrate summits and 

 are separated by rather broad, flat, smooth interspaces ; sometimes the serrations 

 are replaced by numerous transverse, scale-like structures. Near the fixed part 

 of the right valve the ribs are more numerous than on the flank. 



Measurements of fixed val/Ue : 



(i) (2) 



Length . .22 . 19 



Height . 28 . 26 



(1 — 4) Norwich. 

 Affinities. — G. ciplyana (de Ryckholt), 1 from Ciply, differs from this species 

 in having the ribs more widely separated and in the presence of small ribs in 

 the interspaces and on the sides of the main ribs. G. russiensis (d'Orbigny) 3 

 resembles closely G. inequirostrata, and was regarded by Morris as a synonym of 

 the latter, but it possesses small ribs on the sides of the main ribs. 3 



1 'Melanges Pak'ont.,' pt. ii (1851), p. 179, pi. xii, figs. 12, 13; Douville, 'Bull. Soc. geol. de 

 France,' ser. 3, vol. xv (1887), p. 744, pi. xxviii, fig. 11; Holzapfel, ' Molluslc. Aachen. Kreide ' (1889), 

 p. 189, pi. xix, figs. 5, 6; Wollemann, 'Fauna d. Liineburg. Kreide' (1902), p. 75, pi. ii, figs. 5, 6. 



2 Murckison, de Verneuil, and de Kevserling, ' Geol. de la Russie,' vol. ii (1845), p. 496, pi. xliii, 

 figs. 31—33 ; Douville, 'Bull. Soc. geol. de France,' ser. iii, vol. xv (1887), p. 775, pi. xxviii, fig. 13. 



3 G. Miinsteri (v. Hagenow) is probably related to G. inequirostrata. See Bavn, ' Mollusk. i 

 Danmarks Kreidtafl. I, Lamellibr.' (1902), p. 126, pi. iv, figs. 10, 11. 



(3) 



(4) 



16 



14 mm 



20 



18 „ 



