LAMELLIBRANCHIATA. 557 
Vanuxemia hayniana.] 
line. Umbo full, and the whole surface neatly rounded. Outline obliquely acuminate- 
ovoid with the anterior end narrowly rounded and projecting scarcely, if at all, 
beyond the beaks, from which the margin slopes backward with a gentle curve into 
the base; posterior end broad, uniformly rounded; cardinal margin straight, about 
three-fifths as long as the diagonal length of the shell, rounding into the posterior 
margin. Surface with faint wrinkles of growth and probably with finer concentric 
lines. Shell substance thin. Hinge plate rather narrow, with two long posterior 
and two or three short cardinal teeth in each valve. The latter are difficult to see 
because of the closely incurved beaks. Anterior muscular impression, as seen in 
casts of the interior, scarcely visible in a side view, being overhung by the side of 
the Umbo. In an end view they appear like two narrow vertical lobes tapering 
upward and placed just beneath the free portion of the beaks. Posterior scar very 
faint, large, ovate, situated a short distance beneath the extremity of the hinge. 
Pallial line distinct considering the thinness of the shell. 
In the thin shell, its general form, and particularly in the character of the 
anterior muscular impressions, V. terminalis reminds strongly of Ambonychia. It is 
possible that this resemblance is merely coincidental, but I must say that I do not 
believe it, even if I can not now present plausible arguments to show that it 
expresses natural relationship. - As a rule, it is not good policy to speculate in 
paleontological questions, but in the present instance I may be pardoned when I 
state my conviction that the Ambonychiide are an off-shoot from the same line of 
development that produced Vanuxemia and the rest of the Cyrtodontide. 
Seven of the species of Vanuxemia described in this report are found at Minne-~ 
apolis in the same beds that have furnished V. terminalis. All of them occur as 
casts of the interior, yet not one of the others is at all likely to be confounded with 
the present species. The principal peculiarities of the latter are the terminal beaks, 
almost hidden anterior muscle scars, the thin shell and the absence of the internal 
ridge-like thickening which in nearly all species of the genus produces a more or less 
well marked sulcus across the umbonal and anterior parts of casts. 
Formation and locality—Trenton limestone, Minneapolis and Cannon Falls, Minnesota. Also in 
the “Lower Blue beds” of the Trenton near Beloit, Wisconsin. 
Mus. Reg. Nos. 5100, 8320. 
VANUXEMIA HAYNIANA Safford. 
PLATE XXXVIII, FIG. 32. ALSO FIG. 36-VI, P. 479. 
Cyrtodonta hayniana SAFFORD, 1869. Geol. Tenn., pl. F., fig. 1. 
Cypricardites haynianus ULRICH, 1892. Nineteenth Ann. Rep. Geol. and Nat. Hist. Sur. Minn., p. 240. 
Cypricardites triangularis SARDESON, 1892. Bull. Minn. Acad. Nat. Sci., vol. iii, p. 338. 
Shell of medium size, moderately convex, oblique, broadly subovate or obscurely 
quadrate, narrowing anteriorly, the hight and length respectively as nine is to ten; 
