LAMELLIBRANCHIATA. 555 



yanuxemia sardesoni.] 



The species may be compared with V. inconstans Billings, V. niota Hall, sp., V. 

 hayniana Safford, spr, V. sardeson and V. umbonata. None of these forms, however, 

 seem to me sufficiently similar to render the separation of the present species 

 troublesome. 



Formation* and locality.— S\ue limestone of the Trenton at Minneapolis, Minnesota. 

 Mus. Reg. No. 5524. 



' Vanuxemia sabdbsoni Ulrich. 



PLATE XXXVII, FI&S. 17-19, and PLATE XXXVIII, FIG. 45. 



Cypricardites sardesoni, Ulbich, 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 foi;m 

 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 Qonsiderably curved and the lower one forms the upper boundary 

 of the very sharply impressed anterior muscular scar. On the whole the hinge 

 impresses one a^ being unusually strong. The posterior muscular scar is large, 

 ovate, slightly prolonged below and but faintly impressed. 



Comparing casts with the associated V. oUusifrons, 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 tocai%.— Blue limestone cif the Trenton at Minneapolis, Minnesota. 

 Mus. Beg. No. 8335. 



