PECTEN. 215 
1847. Janira Duremp.et, A. d’Orbigny. Pal. Frang. Terr. Crét., vol. iii, p. 646, 
pl. cecexlvuy, figs. 8—11. 
1850. — —_ — Prodr. de Pal., vol. 11, p. 253. 
—  Percren xQuicosratus, J. de C. Sowerby, in F. Dizon. Geol. Sussex, p. 356 
(p. 886, ed. 2), pl. xxviii, 
figs. 17, 18. 
1854. _- sexcosratus, J. Morris. Cat. Brit. Foss., ed. 2, p. 177. 
1870. Janrra Duvempcet, I’. J. Pictet and G. Campiche. Foss. Terr. Crét. Ste. 
Croix (Matér. Pal. Suisse, ser. 5), p. 253. 
1877. — — A. Peron. Bull. Soe. géol. France, ser. 3, vol. v, p. 507, 
pl. vii, figs. 4, 4a. 
1887. Vora (Janrra) Durempet, Peron. L’Hist. Terr. Craie (Bull. Soc. Sci. 
hist. et nat. de l’Yonne, ser. 3, 
vol. xii), p. 164. 
1889. — Dvurempier, O. Griepenkerl. Senon. von Kénigslutter (Paleont. 
Abhandl., vol. iv), p. 48. 
1891. - — J. Bihm. Die Kreidebild. des Fiirbergs u. Sulzbergs 
(Paleontographica, vol. xxxvill), p. 83, 
pl. i, fig. 33. 
1896. — — ?, A. Rutot. Bull. Soc. Belge de Géol., ete., vol. x, 
p. 31, fig. 14. 
?1901-2. — _ cf. Durempxes, P. Choffat. Faune Crét. Portugal, vol. i, ser. 4, 
p. 149, pl. iii, figs. 8, 9. 
Description.—Form (6 (from the Upper Chalk, figs. 1—5). Shell ovately 
triangular, considerably higher than long, nearly equilateral. Ventral margin 
nearly semicircular, but with concave incisions of moderate depth between the 
main ribs. Both valves show well-marked growth-ridges at fairly distinct intervals. 
Hinge-line relatively short. 
Right valve very convex, with six main ribs. Umbo greatly incurved and pro- 
jecting beyond the level of the left valve. Antero- and postero-dorsal areas 
usually sloping inwards and concave. The six main ribs are usually prominent, 
and are separated by broad, concave, more or less deep interspaces. In the inter- 
spaces there are from four to seven ' well-marked rounded ribs separated by furrows 
of about the same width; the central ribs are usually rather stronger than those 
at the sides (near the main ribs). The number of ribs in the interspaces varies 
in different specimens, and usually also in different imterspaces of the same speci- 
men; rarely four ribs are found in each interspace, less rarely five in each, but 
frequently five in four of the interspaces and six or seven in the remaining inter- 
space, or the numbers in different interspaces may be four, five and six, or five, six 
and seven. ‘The antero-dorsal area of this valve (7. e. the space between the anterior 
main rib and the anterior margin of the valve) bears from six to eight narrow ribs 
separated by broader furrows ; these ribs are smaller than those in the mterspaces 
1 Occasionally three or four very much smaller ribs are also present. 
