LAMELLIBRANCHIATA. 555 
Vanuxemia sardesoni.] 
The species may be compared with V. inconstans Billings, V’. niota Hall, sp., V. 
hayniana Safford, sp., V. sardeson and V. umbonata. None of these forms, however, 
seem to me sufliciently similar to render the separation of the present species 
troublesome, 
Formation and locality.—Blue limestone of the Trenton at Minneapolis, Minnesota. 
Mus. Reg. No. 5524. : 
VANUXEMIA SARDESONI Ulrich. 
S PLATE XXXVII, FIGS. 17—19, and PLATE XXXVIII, FIG. 45. 
Cypricardites sardesoni, ULRICH, 1892. Nineteenth Ann. Rep. Geol. and Nat. Hist. Sur. Minn., p. 231. 
Shell a little above the medium size, known only from casts of the interior, and 
the impression of the hinge and free margins on the limestone matrix. The outline 
was subrhomboidal, with the cardinal and anterior margins nearly straight, and the 
two lines forming an angle of about 62°; anterior extremity subacuate or sharply 
rounded, hinge line equaling nearly three-fourths of the entire length, postero-ventral 
margin broadly rounded, almost semicircular; above this the posterior outline is some- 
what straightened and slopes forward rapidly, meeting with the cardinal line to form 
an angle of about 135°; the immediate junction however is not perceptibly angular. 
In the casts the beaks project strongly, are nearly terminal, pointed, slightly 
incurved, greatly compressed, and somewhat twisted. A strong sulcus extends from 
the beaks to the postero-basal part of the casts; this sulcus occupies the larger part 
of the anterior slope, and from its inner side the umbonal ridge, constituting the 
highest portion of the surface, rises abruptly.. For the reasons mentioned the 
anterior slope appears flattened and in part concave, while the posterior is almost 
uniformly convex to the margin. Cardinal slope abrupt, especially near the hinge. 
Gutta-percha impressions bring out the internal characters in a very satisfactory 
manner. They show a wide and faintly striated ligamental area, two lateral and 
two cardinal teeth, both pairs large and distinctly crenulated on the sides. The 
cardinal pair are considerably curved and the lower one forms the upper boundary 
of the very sharply impressed anterior muscular scar. On the whole the hinge 
- impresses one as being unusually strong. The posterior muscular scar is large, 
ovate, slightly prolonged below and but faintly impressed. : 
Comparing casts with the associated V. obtusifrons, which is nearer than any 
other now known, the present species differs in its greater obliquity, narrower 
anterior end, much stronger umbonal sulcus, broader and better defined ligamental 
area, and stronger as well as more coarsely crenulated hinge teeth. 
*. Formation and locality.—Blue iimestone of the Trenton at Minneapolis, Minnesota. 
Mus. Reg. No. 8335. 
