LAMELLIBRANCHIATA. 551 
Vanuxemia dixonensis, var. insueta.] 
shell itself, and a flattening of the surface beneath them is usually distinguishable. 
Anterior muscular impression distinct, reniform, the pair forming a strongly defined 
lobe at the base of the beaks. VTosterior scar large, but very faint. Pallial line 
rather indistinct, except in the anterior part. 
Shell substance very thick in the anterior third. Ligamental area with good 
definition, strongly concave, long, high, but not wide in a dorsal view. Posterior 
lateral teeth, three in each valve, the upper often much the smallest; in many cases 
more nearly horizontal than shown in fig. 4. Cardinal teeth normally three in each 
valve, subequal, nearly horizontal, slightly curved. Occasionally the upper one is 
more slender than usual, and one or both of the others divided so that their number 
may be four or even five in each side. 
This species, which is one of the most abundant and best marked fossils of this 
class found in Minnesota, was at first believed to be identical with V. inconstans Billings, 
but a second comparison with the original description and figure of that spocies 
seemed to throw some doubt upon their identity. This doubt was strengthened to 
conviction when a few days ago I received from Prof. Jas. M. Safford an authentic 
example of Billing’s species. This shows that, despite the close agreement of the 
- two species, Meek and Worthen were fully justified in separating their shell. The 
principal difference lies in the anterior part of the shells, which in V. inconstans is 
more obtuse than in V. dizonensis, and in the upper part just beneath the beaks pre- 
sents a small protuberance where the latter has a lunule-like excavation. ‘This 
difference is due to the shape of the anterior extremity of the hinge, this being 
angular in V. inconstans and well rounded in V. dixonensis. 
Compared with other species, V. rotwndata Hall, sp., and V. suberecta, and V. crassa 
are all less oblique and of rounder outline; in V. obtusifrons the dorsal outline is 
concave instead of convex. 
Formation and locality.—Very common in the upper beds of the Trenton Ilmestone at Minneapolis 
and St. Paul; less abundant at Cannon Falls and other localities in the state. In the ‘“‘ Lower Blue beds” 
at Janesville, Wisconsin, and Dixon, Illinois. 
Mus. Reg. Nos. 202, 320, 670, 5030, 5098, 5525, 5527, 5876, 8322, 8330, 8331. 
VANUXEMIA DIXONENSIS, Var. INSUETA, ”. var. 
PLATE XXXVIII, FIGS, 6 and 7. 
This name is proposed provisionally for one or two casts differing from the 
ordinary form of V. dixonensis apparently in one important respect only, namely, 
the suleus and ridge which should traverse the anterior part of the cast from the 
umbones downward is wanting except above the anterior muscular scar where a 
slight flattening of the umbones may represent the sulcus. The beak also is more 
incurved than in any specimen of the typical form of the species seen. 
