572 THE PALEONTOLOGY OF MINNESOTA. 
(Whitella truncata. 
teeth two in each valve, elongate, slightly curved, nearly horizontal. Muscular sears 
and pallial line faint, not well determined. 
This species is more convex and has a more distinct umbonal ridge than W. 
compressa, and a longer hinge line and larger anterior end than W. megambona, while 
the basal margin is more oblique and the anterior end much larger than in W, sub- 
carinata. 
Formation and locality.—Upper part of middle third of the Trenton shales, St Paul, and near 
Cannon Falls, Minnesota, Also in the Trenton limestones (‘‘ Upper Buff beds”) near Beloit, Wisconsin. 
Wuitetia tTRuNcATA Ulrich. 
PLATE XLI, FIGS. 10—14. 
e 
Whitella truncata ULricu, 1890. Amer. Geol., vol. vi, p. 385. 2 
Shell small, very oblique, ventricose, subrhomboidal in a side view. Beaks 
nearly terminal, prominent, of moderate size, obliquely enrolled; umbones and 
umbonal ridge full, the latter angular and traceable to the postero-basal angle. 
Cardinal slope sharply defined and distinctly concave; anterior and basal slopes 
slightly convex and very rapid. Anterior end very short, scarcely projecting beyond 
the beaks, narrowly rounded, then sloping rapidly backward and uniting very gradu- 
ally with the gently curved basal margin. Posterior end truncated, straightened, 
forming nearly a right angle with tie hinge line, and one of from 75° to 80° with 
the ventral edge. Escutcheon narrow, not extending anterior to the beaks. In casts 
of the interior, the internal cartilage support leaves two narrow impressions, one on 
each side of the posterior half of the hinge line. Dentition of hinge not observed. 
Muscular scars very faint. 5 
Dimensions of a large cast of the interior: Greatest hight, 15 mm.; greatest 
convexity (near center of shell), 15 mm.; length from beaks to postero-basal angle, 
19 mm.; length from anterior extremity to upper portion of posterior margin, 15 mm. 
In a small specimen only 6 mm. high, the other dimensions are in proportion, except 
that the convexity is comparatively less. 
This species is closely related to W. scofieldi, but may be distinguished by its 
smaller size, greater convexity, truncated posterior end, shorter anterior end: and 
more pronounced postero-ventral angle. 
Formation and locality.—Galena shales, Goodhue county, Minnesota. 
WHITELLA SUBCARINATA, 2. Sp. 
PLATE XLI, FIGS, 22 and 723. 
This species is in many respects like W. truncata, but is readily distinguished by 
its lesser gibbosity, smaller beaks and more rounded shape. The umbonal ridge is 
