556 THE PALEONTOLOGY OF MINNESOTA. 



[Vanuxemla umbonata. 



Vanuxemia umbonata, n. sp. 



PLATE XXX VIII, FIGS. 28-31. 



Shell' of medium size, tumid in the rostral and central parts, the hight about 

 one-seventh greater than the length; obliquely subovate, hinge line rather short, the 

 anterior extremity stibangular and projecting a short distance beyond the beaks. 

 Anterior margin gently convex, vertical, rounding neatly into the semicircular base; 

 posterior margin broadly convex, the junction with the hinge line obtusely angular, 

 Umbones evenly tumid, very prominent, the beaks curving forward and down to the 

 hinge. Cardinal slope, ccJncave; postero-cardinal portion of shell compressed. Sur- 

 face not well preserved in any of the specimens seen, apparently marked with rather 

 strong and somewhat irregular concentric lines of growth. Shell substance compara- 

 tively thin, so that the internal rostral and anterior thickening produces but a very 

 obscure sulcus on internal casts. Anterior muscular scar sharply defied, reniform, 

 of good size; posterior scar not observed; pallial line distinct in the anterior and 

 basal parts. Hinge plat^ rather strong, with a narrow ligamental area posterior to 

 the beaks; cardinal teeth long, nearly horizontal though distinctly curved, two in the 

 right valve; posterior lateral te^eth four in the right valve, slender, oblique. 



This species is doubtless closely allied to V. obtusifrons but may be distinguished 

 at once by its thinner shell, the greater projection of the anterior extremity of the 

 hinge, and the greater length and more nearly horizontal arrangement of the cardi- 

 nal teeth. Of the latter also there are only two instead of three, and they are not 

 crenulated as in the species. The posterior teeth again are more slender.' V. hayniana 

 Safford, sp., is shorter and has a longer hinge line. One of the specimens is imper- 

 fect, so that it resembles Cyrtodonta cingulata, a rare species, occurring in the same 

 beds, and having similar surface markings. However, a comparison of external 

 characters alone reveals sufficient difference to render confusion between them highly 

 improbable, especially when the possibility of such an occurrence is borne in mind. 

 The hinge line of the Cyrtodonta, namely, is longer, the shell is more erect, the ante- 

 rior end much longer, and the umbones, though more, strongly convex, are on the 

 whole much less tumid. 



Formation and locality.— Vpjpei part of the mddle third of the Trenton shales, Minneapolis and St. 

 Paul, Minnesota. Also in the Black Eiver horizon of the Trenton formation in Mercer county, Kentucky. 



Vanuxemia terminalis Ulrich. 



PLATE XXXVIII, FIGS. 33 and 34. 



Cyprieardites terminalis Ulkich, 1892. American Geologist, vol. x, p. 98. ' 



Shell of medium size, moderately ventricdse, extremely oblique, with the beaks 

 terminal, rather small, strongly incurved and projecting but little above the hinge 



