PTYCHOMYA. 179 



Family— VENERID^E, Leach. 



Genus — Ptychomya, L. Agassiz, 1842. 

 (' Etudes crit. Moll. Foss.' ; Myes, p. xviii, pi. xi, figs. 3, 4.) 



Ptychomya Rocinaldina (d'Orbigny), 1844 Plate XXVII, figs. 24—20. 



1842-45. Ptychomya plana, L. Agassiz. Etudes crit. Moll. Fuss. ; Myes, p. xviii 



(1845), pi. xi, figs. 3, 4 (1842). 



1844. Ciiassatella ltoiiiNALDiNA, A. d'Orbigny. Pal. Frauc. Terr. Oct., vol. iii, 



p. 75, pi. cclxiv, figs. 10 — 13. 



1845. E. Forbes. Quart. Jouru. Geol. Soc, vol. i, 



p. 241. 

 1850. d'Orbigny. Prodr. de Pal., vol. ii, p. 77. 



1854. J. Morris. Cat. Brit. Foss., ed. 2, p. 196. 



1855. G. Gotteau. Moll. Foss. de l'Yonne, p. 71. 

 1800. Ptychomya F. J. Pictet and G. Campiche. Foss. Terr. Cret. 



Ste. Croix (Mater. Pal. Suisse, 

 ser. 4), p. 352, pi. cxxvii, 

 figs. 2 — 6. 

 1809. Eadioconcha T. A. Conrad. Amer. Jouru. Couch., vol. v, 



p. 47. 



Description. — Shell oval-oblong, elongate, convex, but with flattened sides, very 

 inequilateral, anterior part rather higher than the posterior part. Anterior 

 margin regularly rounded. Ventral margin only slightly curved, nearly parallel 

 to the dorsal margin. Posterior margin oblique, forming an acute angle with the 

 ventral margin and an obtuse angle with the slightly curved postero-dorsal margin. 

 Umbones inconspicuous. On the postero-dorsal side of a line from the umbone to 

 the posterior extremity the shell is flattened. Lunule narrow, elongate. 



Ornamentation consists of strong radial ribs, which are sometimes more or less 

 nodose. Those on the posterior part of the shell are stronger and separated by 

 broader furrows than those on the anterior part. The ribs which reach the 

 posterior margin start from the umbo; the others start from a line between the 

 umbo and a point on the opposite margin which is about a quarter of the distance 

 from the anterior to the posterior end. In front of this line the ribs form a 

 considerable curve ; behind it they are only slightly curved ; the ribs meet at an 

 acute angle at the line mentioned forming either a A throughout or a chevron M, 

 on the ventral part. The postero-dorsal margin is ornamented with strong nodose 

 ridges. 



