206 CRETACEOUS LAMELLIBRANCHIA. 



L866. Cahdium Gentianum, Pictet and Campiche. Ibid., p. 269. 

 1871. peoboscideum, F. Stoliczka. Palaeout. Indica, Cret. Fauna S. 



India, vol. iii, p. 213 (Acantho- 

 cardiv/m). 

 . Gentianum, Stoliczka. Ibid., p. 213 (Acanthocardium). 

 1882. — proboscideum, P. de Loriol, Gault de Cosne, p. 91, pi. xi, fig. 4. 



1900. — Gentianum, E. T. Newton and A. J. Jukes-Browne. In Jukes- 



Browne, Cret. Rocks of Britain, 

 vol. i, p. 448. 



Description. — Shell stout, very convex, oval, higher than long, slightly inequi- 

 lateral. Anterior margin rounded ; posterior margin truncated, forming an 

 angle with the postero-dorsal margin. Umbones prominent. 



Ornamentation consists of radial ribs separated by narrow grooves ; the 

 stronger ribs bear prominent, angular, laterally compressed tooth-like projections, 

 which may be rather larger near the posterior margin than elsewhere ; in the 

 spaces between the stronger ribs are two (sometimes one or three) smaller ribs with 

 similar but smaller tooth-like projections. Margins of valves toothed. 



Measurements : 



(1) (2) (3) (4) (5) 



Length 63 . 59 . 58 . 57 . 47 mm. 

 Height 74 . 70 68 . 65 . 54 „ 



(1 — 5) Blackdown. 



Affinities. — A form from the Cenomanian was referred by d'Orbigny to 

 G. productum, Sowerby, 1 the type of which comes from the Senonian of Grosau. 

 This identification has been accepted by Zittel, Holzapfel and others, but not by 

 Pictet and Campiche, and de Loriol. The specimen figured by d'Orbigny 2 is 

 probably an example of C. proboscideum, Sowerby, and differs from G. productum 

 in the distinct differentiation of the ribs into a larger series separated by smaller 

 series. 



The differences between G. Gentianum and G. proboscideum seem to be due 

 entirely to their different modes of preservation. The former is found in the 

 Upper Greensand of Devizes and Ventnor ; the shell is absent, but the sand which 

 filled the interior of the shell now forms a natural cast of the exterior, showing- 

 more or less imperfectly the character of the ornamentation ; usually the spines 

 are represented by stumps only or are almost completely obliterated. In these 

 specimens the original form of the shell has been more or less considerably 



1 'Trans. Geol. Soc.,' ser. 2, vol. iii (1832), p. 417, pi. xxxix, fig. 15 ; Goldfuss, ' Petref. Germ.,' 

 vol. ii (1837), p. 221, pi. cxliv, fig. 7 ; Zittel, ' Bivalv. d. Gosaugeb.,' pt. i (1864), p. 37, pi. vi, fig. 1 ; 

 Holzapfel, " Die Mollusk. Aachen. Kreide " (' Palseontographica,' vol. xxxv), p. 179, pi. xvii, figs. 1 — 5 ; 

 G. Miiller, ' Mollusk. d. Untersen. v. Braunschweig u. Ilsede ' (1898), p. 63, pi. ix, figs. 13, 14. 



2 'Pal. Franc. Terr. Cret.,' vol. iii (1844), p. 31, pi ccxlvii ; Gueranger, 'Album Palcont. de la 

 Sarthe' (1867), p. 15, pi. xx, figs. 8—10. 



