558 THE PALEONTOLOGY OF MINNESOTA. 
[Vanuxemia hayniana. 
hinge line nearly straight, rather long, terminating subangularly behind; posterior 
margin broadly rounded, slightly oblique, generally forming, with the basal margin 
a semicircle drawn to a diameter but little shorter than the length of the shell, and 
equalling the greatest hight; anterior end projecting very little beyond the beaks, 
nicely rounded from the end of the hinge, sloping backward in the lower half and 
passing very gradually into the basal line. Beaks only moderately prominent, 
strongly incurved, approximate; umbones full, uniformly convex to the beginning 
of the faintly concave dorsal and posterior slopes. Surface marked with more or 
less obscure and unequal concentric lines, some of them often, especially near the 
margin of old shells, being of a strongly lamellose character. 
Hinge of moderate strength, the plate varying between 2 and 3 mm. in width at 
the middle in adult specimens; about half of the width taken up by a long and very 
finely lined ligamental area. Posterior lateral teeth constantly three in each valve, 
nearly straight, ranging at an angle of about 40° with the hinge line. Cardinal teeth 
varying in number and size. As a_rule they are at least 10° more oblique than the 
posterior ones and normally of nearly equal size, finely toothed or striated and three 
in each valve. The variations are evidently due to irregularity of development, 
Rarely there are two large ones with a small one on each side; more commonly one. 
two, or even all three will be divided, so that the total number may reach six. 
Anterior muscular scar sharply defined, semicircular; posterior scar ovate, as usual 
very faintly impressed; pallial line quite distinct, except in the posterior part. 
Umbonal cavity small, compressed; anterior internal thickening of the valves gen- 
~ erally rather sharply defined on the inner side. 
Casts of the interior, the only condition in which the species has been found in 
Minnesota, have small compressed pointed and scarcely incurved beaks, projecting 
slightly beyond the hinge line and situated farther behind the anterior extremity of 
the shell than is the case on the exterior. The ridge immediately behind the 
anterior flattening or sulcus is well marked, as is also the pallial line and the ante- 
rior muscular scar. Indeed the natural casts correspond exactly with artificial ones 
prepared from typical Tennessee and Kentucky specimens of the species. 
This is a widely distributed and well marked form, about which a number of 
closely related species or varieties are grouped. One of these, V. subrotunda, occurs 
in Minnesota, but in lower beds than the typical form. It is distinguished by its 
more circular outline. Another, V. abrupta, from the Galena of Fillmore county, is 
more easily separated by its more nearly terminal beaks, very obtuse anterior side, — 
thinner shell, and in wanting the ridge which marks the casts of V. hayniana. A 
third form is found in the upper Trenton of Kentucky. Being a smaller shell I - 
called it nana (Cypricardites nanus Ulrich, 1892. Nineteenth Ann. Rep. Geol, and Nat. 
