946 THE PALEONTOLOGY OF MINNESOTA. 



[Omosplra alexandra 



more or less completely closing the latter in all save one of the specimens before 

 us. Surface marked with fine, regular, equal, raised lines of growth. These curve 

 rather strongly backward on the flattened upper portion of the whorls until they 

 approach the shoulder-like angulation, when they turn sharply forward. About 

 midway between the suture line and the outer angle, the growth lines are interrupted 

 by a sharply elevated revolving line, causing the outer half of the upper slope to 

 resemble an unusually wide slit-band. Beneath the angle the lines descend with a 

 gentle forward curve to the base of the whorl. 



This fine species is readily distinguished from 0. alexandra Billings sp., by its 

 higher and more angular whorls. 



Formation and locality. fop of Stones River group or base of the Black River group, near Leb- 

 anon, Tennessee. 



Collections. Plot. J. M. Safford; E. O. Ulrich. 



OMOSPIRA ALEXANDRA Billings. 



PLATE LXX, FIGS. 66 and 67. 



Murchisonia ventricosa SALTER, 1859, Can. Org. Rem., Decade 1, p. 23. (Not Murchisonia ventricosa 

 Hall, 1847. 



Murchisonia alexandra BILLINGS, 1865, Pal. Foss., vol. i, p. 172. 







This species has the same general characteristics as the preceding, yet may be 

 distinguished at once by its more depressed and more rounded volutions. Its apical 

 angle also is narrower, being between 40 and 45, while the aperture is more 

 rounded. There is not a sign of an umbilical perforation. 



The cast of the interior figured as possibly of this species may in reality belong 

 to something quite different. At any rate, it is the only fossil from the north- 

 western states which we could say probably represents this species. 



Formation and locality. Base of the Trenton group or at the top of the Black River group, in 

 Mercer county, Kentucky. The types are from the latter horizon at Allumette island in the Ottawa river. 

 Canada. 



Collection. E. O. Ulrich. 



Family PLEUROTOMARII1LE, d'Orbigny. 



This large and most important family of fossil shells has given systematists not 

 a little trouble to classify. The great number and variety of the species has 

 occasioned many attempts to arrange them in convenient generic and subgeneric 

 groups, sometimes happily, but in most cases the result proved neither convenient 

 nor successfully defensible when subjected to the test of genetic relationship. As 

 usual many of the subdivisions as drawn by authors were necessarily wrecked 

 through the prevailing ignorance concerning the structural peculiarities and lines 

 of development exhibited by the Paleozoic types, the early Paleozoic especially. 



