572 THE PALEONTOLOGY OF MINNESOTA. 



• [Whitella truncata. 



teeth two in each valve, elongate, slightly curved, nearly horizontal. Muscular scars 

 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. megamhona, while 

 the basal margin is more oblique and the anterior end much larger than in W, sub- 

 carinata. 



Formation and iocaZ%.— Upper part of middle third of the Trenton shales, St Paul, and near 

 Cacnon Falls, Minnesota, Also in the Trenton limesjiones (" Upper Buffi beds") near Beloit, Wisconsin. 



Whitella truncata Ulrich. 



PLATE XLi; FIGS. 10-14. 



Whitella truncata Ulkich, 1890. Amer. Geol., vol. vi, p. 385. 



Shell small, very oblique, ventricose, subrhomboidal in a side view. Beaks 

 nearly terminal, promine!nt, 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 the 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. 



Dimensions of a large cast of the'interior: Greatest hight, 13 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. scqfieldi, 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 suboarinata, n. sp. 



PLATE XLI, FIGS. 22 and ?23. 



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 



