972 THE PALEONTOLOGY OF MINNESOTA. 



[Lophosplra. perangulata. 



separating them. In the first place, the free whorls of L. serrulata have only four 

 carinse, being entirely without the uppermost or sutural keel which occurs con- 

 stantly in L. helicteres. Then the surface striae in crossing the shell from keel to 

 keel in the latter are directed very much less backward and forward than is the 

 case in the former. The outer edge of the aperture is therefore quite different in 

 the two species. (Compare figs. 26 and 55 on pi. LXXII.) With practice it is 

 possible to distinguish them almost at a glance, and it is not by any means a hope- 

 less task even when we have nothing but the casts of the interior. When the casts 

 are entire at the aperture, L. serrulata is recognized by the projecting angles at the 

 extremities of the first and third carinse and the wide >shaped notch between them. 

 When this test is not available then we must rely upon the relative distinctness of 

 the carinae on the upper whorls. They are recognizable much farther up on the 

 spire in L. helicteres than in L. serrulata, providing, of course, the specimens are of 

 equal size. 



It is possible that the shell above described is the one which Conrad named 

 M. tricarinata, but Halls figures of that species are so poor that we cannot be blamed 

 if we have made a synonym. 



Formation and locality. Stones River group, Minneapolis and St. Paul, Minnesota; Mineral Point, 

 Janesville and Beloit, Wisconsin; and Dixon, Illinois. The typical form is from the Black River group 

 at Pauquette's rapids, Ottawa river, Canada. We have it also from a similar horizon in Mercer county, 

 Kentucky. It is said to occur also in the Upper Buff limestone in Wisconsin. 



Collections. Geological and Natural History Survey of Minnesota; E. O. Ulrich. 

 Museum Register, No. 6858, 7360. 



LOPHOSPIRA PERANGULATA Hall. 

 PLATE LXIII, FIGS. 1-7. 



Murchisonia perangulata HALL, 1847, Pal. N. Y., vol. i, p. 41, pi. x, flg. 4; not p. 179, pi. xxxvm, flgs. 

 7a, 76; SALTBR, 1859, Can. Org. Rem., decade 1, p. 19, pi. iv, flg. 7. 



Shell small, hight 10 to 20 mm.; apical angle usually about 52, but varying 

 between 50 and 57". Volutions about six, the last inclining to become free, 

 scarcely ventricose below, very gently concave above; peripheral band prominent, 

 sharp, triliueate; lower carina distinct though not very prominent, sometimes very 

 obscure on casts of the interior; upper slope without a carina, the gentle concavity 

 extending to the suture. A small, abruptly defined umbilicus always present. 

 Mouth subtriangular, slightly drawn out below. Surface markings consisting of two 

 sets of strongly recurved lines of growth, one distant and sublamellose, the other 

 much finer and closely arranged between the former. 



We have every reason to believe that the shell above described is identical with 

 the Birdseye type of the species, but it is not the same as the Trenton form which 



