v,„, , ^ , LAMELLIBRANCHIATA. 557 



vanuxemla hayulaua.] 



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, scai:cely 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, F. ierminalis 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 Amhonychiidce are an off-shoot from the same line of 

 development that produced Vanuxemia and the rest of the Cyrtodontidce. 



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. Beg. Nos. 5100, 8320. 



Vanuxemia hayniana Safford. 



PLATE XXXVIIl, FIG. 32. ALSO FIG. 36- VI, P. 479. 



Cyrtodonta hayniana Saffobd, 1869. Geol. Tenn., pi. F., flg. 1. 



Cypricardites haynianus Ulbich, 1893. Nineteenth Ann. Rep. Geol. and Nat. Hist. Sur. Minn., p. 240. 



Cypricardites triangularis Sabdeson, 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; 



