DREISSENSIA. 1 
1854. Myrinus rripens, Morris. Ibid., p. 215. 
1858. — LANCEOLATUS, I’. J. Pictet and EB. Renevier. Foss. Terr. Aptien 
de la Perte du Rhone (Mat¢r. Pal. Suisse, ser. 1), 
p: L110; pl. xv; fig: 7: 
r 1861. — Gaviiennel, H. Trautschold. Bull. Soc. Imp. Nat. de Moscou, 
1867. -- LANCEOLATUS, Ff’. J. Pictet and G. Campiche. Foss. Terr. Crct. 
Ste. Croix (Matcr. Pal. Suisse, ser. 4), p. 485. 
1868. oa — A, Briart and F. L. Cornet.  Descript. Meule 
de Bracquegnies (Mém. cour. et Mém. Sav. 
¢étrangers, vol. xxxiv), p. 52, pl. iv, figs. 11, 12. 
vol. xxxiv, pt. 2, p. 433, pl. xii, fig. 4. 
? 1868. Moprona susraucara, L. Vichwald. Lethea Rossica, p. 533, pl. xxi, fig. 14. 
1871. Myrintus tancrouatus, J. Stoliczka. Paleont. Indica, Cret. Fauna S. 
India, vol. ili, p. 372. 
1872. _ _- EF’. Schmidt. Mammuthexpéd. (Mcém. de Acad. 
Imp. Sci. de St. Pétersb., vol. xvii, No. 1), 
p. 154, pl. ii, fig. 7; pl. iii, fig. 12. 
y 1884. _ a J. F. Whiteaves. Geol. and Nat. Hist. Survey of 
Canada: Mesozoic Fossils, vol. i, pt. 3, p. 236, 
pl. xxxi, fig. 7. 
Non 1868. Moprona Lancronara, HL. Kichwald. Lethea Rossica, p. 532, pl. xxii, 
fig. 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 curving 
gradually at the posterior extremity of the valve. Umbones sharp, terminal ; 
apical angle varying from 47° to 53°—average 513°. 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 m 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 near the cara ; 
sometimes also cn the antero-ventral margin. Interior not seen except at the 
umbonal region; wmbonal septum present ; teeth absent or rudimentary. 
