DREISSENSIA. Ill 



1854. Mytilus tridens, ilorris. Ibid., p. 215. 



1858. — LANCEOLATUS, F. J. Pictet and E. Benevier. Foss. Terr. Aptien 



de la Perte du Rhone (Mater. Pal. Suisse, ser. 1), 



p. 110, pi. XV, fig. 7. 

 ? 1861. — Galliennei, H. TrautscJwld. Bull. Soc. imp. Nat. de Moscou, 



1867. — LANCEOLATUS, F. J. Pictet and G. Canipiche. Foss. Terr. Cn't. 



Ste. Croix (Mater. Pal. Suisse, ser. 4), p. 485. 



1868. — —A. Briart and F. L. Cornet. Descript. Meule 



de Bracquegnies (Mem. cour. et Mem. Sav. 

 etrangers, vol. xxxiv), p. 52, pi. iv, figs. 11, 12. 

 vol. xxxiv, pt. 2, p. 433, pi. xii, fig. 4. 

 ? 1868. MoDioLA suBFALCATA, E. Eickicald. Lethsea Rossica, p. 533, pi. xxi, fig. 14. 



1871. Mytilus lanceolatus, F. Stoliczka. Pala^out. Indica, Cret. Fauna S. 



India, vol. iii, p. 372. 



1872. — — F. Schmidt. Mammuthexped. (Mem. de I'Acad. 



Imp. Sci. de St. Petersb., vol. xviii. No. 1), 

 p. 154, pi. ii, fig. 7 ; pi. iii, fig. 12. 

 ? 1884. — — J. F. Whiteaves. Geol. and Nat. Hist. Survey of 



Canada: Mesozoic Fossils, vol. i, pt. 3, p. 236, 

 pi. xxxi, fig. 7. 



Non 1868. Modiola lanceolata, E. Eichwald. Lethsea Eossica, p. 632, pi. xxii, 



fio'. 5. 



Description. — Shell stout, convex; outline subtrapezoidal, or sometimes sub- 

 triangular. Hinge-margin slightly convex, sometimes straight, usually nearly half 

 the length of the antero- ventral margin ; the latter is slightly concave. Posterior 

 margin convex, often roughly parallel to the antero-ventral margin, and cui-ving 

 gradually at the posterior extremity of the valve. Umbones sharp, terminal ; 

 apical angle varying from 47° to bo° — average 51^°. Carina sharp, but rather 

 more rounded on the older parts of the shell ; usually close to the margin except in 

 the umbonal region. The shell in front of the carina is vertical, or nearly vertical, 

 to the plane of the valves, except near the umbones, where it slopes outward. 

 Behind the carina the shell slopes gradually to the posterior and dorsal margins. 

 Ornamentation : the greater part of the shell is usually nearly smooth except for 

 growth-lines, but concentric ribs are seen on the earlier parts and neai' the carina ; 

 sometimes also on the antero-ventral margin. Interior not seen except at the 

 umbonal region ; umbonal septum present ; teeth absent or rudimentary. 



