LAMELLIBEANCHIATA. 551 



Vannxemia dlxonensis, var. insueta.l 



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. Posterior 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 valye, 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. Saflford 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. dlxonensis, 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. rotundata Hall, sp., and V. suberecta, and F. crassa 

 are all less oblique and of rounder outline; in V. ohtusifrons the dorsal outline is 

 concave instead of convex. 



Formation and looalify.—YeTy common in the upper beds of the Trenton limestone at Minneapolis 

 and St. Paul; less ahundant at Cannon Falls and other localities in the state. In the " Lower Blue beds " 

 at Janesville, Wisconsin, and Dixon, Illinois. 



Mus. Beg^. Nos. 202, 320, 670, 5030, 5098, 5525, 5527, 5676, 8322, 8330, 8331. 



V-ANuxEMiA DIXONENSIS, var. iNsuETA, w. 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 sulcus 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. 



