550 THE PALEONTOLOGY OF MINNESOTA. 



[Vaiui.xeintii dixoiiensis. 



in casts of the interior forming a distinct lobe-like prominence, often of reniform 

 shape, immediately in front and sometimes partly between the filling of the beaks. 

 Posterior scar indistinct, larger than the anterior. Pallial line simple. Internal 

 umbonal ridge well developed. 



Type: Vanuxemia inconstans Billings. 



As a rule this genus can be distinguished from Cyrtodonta by the more nearly 

 terminal position and greater prominence of the beaks, but the final and only reliable 

 test lies in the position and character of the anterior adductor scar. -This, in being 

 excavated out of the hinge plate instead of being placed on the floor of the valve, 

 makes so obvious a difference that I cannot see, now that it is pointed out, how any 

 one can fail to discriminate between the two genera. 



Between twenty and twenty-five valid species of Vanuxemia are known to me. 

 They are all Lower Silurian and, although Billings has placed a Devonian shell here, 

 I am almost satisfied that the genus became extinct with the close of the Hudson 

 River deposits. 



VANUXEMIA DIXONENS-IS Meek and Worthen. 



PLATE XXXVIII, FIGS. 1-5. 



Vanuxemia dixonensis MEEK AND WORTHEN. Pro. Chicago Acad. Sci., vol. i, p. 16; also 1868, Geol. 



Sur. 111., vol. ill, p. 297, pi. 1, flg. 5a, b. 



Shell beneath medium size, very gibbous, obliquely acuminate-ovate, the nar- 

 rowly rounded rostrum forming the small end of the oval. Outline gently arcuate 

 dorsally, and usually rather sharply rounded at the posterior extremity of the hinge; 

 from this point around the lower half of the shell, the outline sometimes forms a 

 regular semicircle, but it is more common to find that the center of the base is more 

 or less produced. (See fig. 4.) Anterior end rounded, projecting very little, if at all, 

 beyond the nearly terminal beaks. Beneath the latter the outline is insinuated 

 often strongly, but in most cases more gently than in fig. 5; in a front view an unde- 

 fined heart-shaped lunule-like depression. Umbones tumid and prominent, with the 

 beaks curving strongly inward and forward. An obtuse curving ridge extends from 

 each beak backward along the depressed hinge line. These dorsal ridges form a 

 broad flattened or rather concave" back to the closed valves. Just within them an 

 impressed line, defining a lanceolate escutcheon-like area, is sometimes distinguish- 

 able. Surface marked with strong, but unequal concentric lines of growth. 



In casts of the interior the beaks stand far apart (much more so than in the 

 shell), are very prominent, broad, much compressed, concave on the inner side, sharp- 

 edged in front and very little incurved, while a more or less strong and nearly vertical 

 sulcus and ridge marks the anterior half. The dorsal ridges are shaiper than < r. th 



