17(5 ('HETA(^EOUS LAMELLIBUANCHIA. 



? lbl'7. Pecten CEETOSUS, var. NiTiuA, ^. Hctin!(j. Revis. af LaiiU'l]. i Nilssou's 



' Potrif . Suecaua ' (Kon. Fjsiogr. Siillsk. 



i Lund. Haudl., N. F., vol..viii), p. 49. 



1898. — — G. Midhr. Mollusk. d. TJntersenou v. Braunschweig, 



etc. (Abliand. d. k. preussisch. geol. Lande- 

 saust., N. F., Heft 25), p. 31, pi. v, fig. 1. 

 1900. — — var. Zeisneki, C. Gagel and F. KatinJiowen. Jalirb. d. 



k. preussisch. geol. Landesaust. u. 

 Bergak. f iir 1899, p. 229. 

 — — — A. WoUcmann. Die Fauna Senons von Biewende 



(ibid., 1900), p. 16. 

 1902. — — J. P. J. Itavn. Mollusk. i Danniarks Kridfafi. I. 



Lamellibr. (K. Danske Vid. Skrift. 6 

 Eaekko, nat. math. Aid., vol. xi), 

 p. 88, 1.1. i, figs. 11, 1,S. 

 •^ --- -- var. MTiDA, Ert(;;(. Ibid., p. 88, j)!. i, figs. 12, 13, 21. 



Non 1833. — — A. Goldfuxs. Petref. Germ., vol. ii, p. 58, pi. xciv, fig. 2 



( = critij)us, Romer, and elongatvs, Lamarck). 



Besciijifiuii. — Shell thin, ovate, higher than long, slightly inequilateral, rounded 

 ventrally, antero-dorsal margin slightly concave, postero-dorsal margin straight or 

 sliglitly convex. Valves of slight convexity. Apical angle varying from 90° to 

 105°. Ears moderately large, nnequal. 



Right valve flattened or slightly convex, with numerous narrow ribs which 

 may be very slender or moderately strong; the interspaces are sometimes broader 

 than the ribs, but narrower when the ribs are more numerous. In the interspaces 

 are numerous regularly placed, linear, concentric ridges, which may l)e confined to 

 the neighbourhood of the umbo, or may cover a larger part, or even the entire 

 surface of the valve; these ridges are usually closer together ventrally than near 

 the umbo. At some distance from the umbo, in a few or in many of the inter- 

 spaces, new ribs appear, and sometimes remain throughout smaller than the 

 primary ribs, but in other cases rapidly become of the same size as the primaries. 

 Near the antero- and postero-dorsal edges of the valve the radial ribs are absent, 

 and numerous fine strise are placed nearly perpendicular to the edge. The ribs 

 hear numerous spiny processes, which are usually scale-like and placed transversely, 

 but may be more poiiiied or roimded and nodular; these processes may occur over 

 the entire surface^ oi' l)e confined to parts, and they vary in size on different 

 specimens. Anterior ear long, Avith a deep sinus; dorsal portion smooth, but 

 between that and the sinus are from three to five spiny ril)s, which are usually 

 rather indistinct. J'osterior ear smallei-, triangulai-, \\ith the outt'r angle slightly 

 obtuse ; with seven or eight radial ribs bearing scaly or nodular |)rocesses, and 

 sometimes crossed by concentric ridges. 



