2UG CHKTACEUU8 LAMEI;L1BRA^XM1IA. 



I80(j. CaIvUIUM CJentianum, Pldd and Camplche. iliid., p. 2G9. 

 1871. — PEOBOSCIDEUM, F. StoUczka. Palseont. ludica, Grot. FLUiiia S. 



India, vol. iii, p. 213 {Acatdho- 

 cardinm). 

 — — Gentianum, StoUczJca. Ibid., p. 213 {Acanthocardium). 



1882. — PEouosciDEUM, P. de Loriul, Gault de Cosue, p. 91, pi. xi, tig. 4. 



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



Bi'owue, Cret. Rocks of Britain, 

 vol. i, p. 448. 



Descrijifhin. — Sliell stout, very convex, ovul, higher than long, slightly inequi- 

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

 angle with the po.stero-dorsal margin. Unil)ones prominent. 



Ornamentation consists of radial ribs separated Ij}- narro^Y grooves ; the 

 stronger ribs bear prominent, angular, laterally compressed tooth-like ))rojections, 

 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) 



Length G3 . 59 . 58 



Height 74 . 7*t . 68 



(1—5) Blackdowu. 



Affinities. — A form from the Cenomanian was referred l)y d'Orbigny to 

 C. pwdnrtum, Sowerby,^ the type of which comes from the Senonian of Gosau. 

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

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

 probably an example of G. fvohoscideum, Sowerby, and differs from G. prothidum 

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

 series. 



The differences between G. Gentiamun and C. prohosc'ideum 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 whicli 

 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 l)y stumps only or are almost completely obliterated. In these 

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



• '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), j). 37, pi. vi, fig. 1 ; 

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

 G. Miillcr, 'Mollusk. d. Uutersen. v. Braunscliweig u. Ilsede ' (1898), p. (33, pi. ix, figs. 13, 14. 



- ' Pal. Frany. Ti'rr. Cret.,' vol. iii (1844), p. 31, pi i-cxlvii ; Gucranger, ' Album Pak'ont. du la 

 Sarlhe ' (1867), p. 15, pi. xx, tigs. 8—10. 



