556 THE PALEONTOLOGY OF MINNESOTA. 
[Vanuxemia umbonata. 
VANUXEMIA UMBONATA, %. Sp. 
PLATE XXXVIII, FIGS. 28—31. 
Shell of medium size, tumid in the rostral and central parts, the hight about 
one-seventh greater than the length; obliquely subovate, hinge line rather short, the 
anterior extremity subangular and projecting a short distance beyond the beaks. . 
Anterior margin gently convex, vertical, rounding neatly into the semicircular base; 
posterior margin broadly convex, the junction with the hinge line obtusely angular. 
Umbones evenly tumid, very prominent, the beaks curving forward*and down to the 
hinge. Cardinal slope, concave; postero-cardinal portion of she!l compressed. Sur-° 
face not well preserved in any of the specimens seen, apparently marked with rather 
strong and somewhat irregular concentric lines of growth. Shell substance compara- 
tively thin, so that the internal rostral and anterior thickening produces but a very 
obscure sulcus on internal casts. Anterior muscular scar sharply defied, reniform, 
of good size; posterior scar not observed; pallial line distinct in the anterior and 
basal parts. Hinge plate rather strong, with a narrow ligamental area posterior to 
the beaks; cardinal teeth long, nearly horizontal though distinctly curved, two in the 
right valve; posterior lateral teeth four in the right valve, slender, oblique. 
This species is doubtless closely allied to V. obtusifrons but may be distinguished 
at once by its thinner shell, the greater projection of the anterior extremity of the 
hinge, and the greater length and more nearly horizontal arrangement of the cardi- 
nal teeth. Of the latter also there are only two instead of three, and they are not 
crenulated as in the species. The posterior teeth again are more slender. V. hayniana 
Safford, sp., is shorter and has a longer hinge line. One of the specimens is imper- 
fect, so that it resembles Cyrtodonta cingulata, a rare species, occurring in the same 
beds, and having similar surface markings. However, a comparison of external 
characters alone reveals sufficient difference to render confusion between them highly 
improbable, especially when the possibility of such an occurrence is borne in mind, 
The hinge line of the Cyrtodonta, namely, is longer, the shell is more erect, the ante- - 
rior end much longer, and the umbones, though more strongly convex, are on the 
whole much less tumid. ; 
Formation and locality.—Upper part of the mddle third of the Trenton shales, Minneapolis and St. 
Paul, Minnesota. Also in the Black River horizon of the Trenton formation in Mercer county, Kentucky. 
VANUXEMIA TERMINALIS Ulrich. 
PLATE XXXVIII, FIGS. 33 and 34. 
Cypricardites terminalis Uiricu, 1892. American Geologist, vol. x, p. 98. 
Shell of medium size, moderately ventricose, extremely oblique, with the beaks 
terminal, rather small, strongly incurved and projecting but little above the hinge 
