996 THE PALEONTOLOGY OP MINNESOTA. 



[Llosplra ainericana. 



then the unusual prominence and the sharpness of the curve of the central portion of the outline of 

 the lower lip. The angularity of the margin of the umbilicus is of course much less marked in casts 

 of the interior than on the shell itself. Still it is always indicated with sufficient clearness to be unmis- 

 takable to the trained observer. For comparisons see following species. 



Formation and locality. Ranges from the base of the Stones River group to the Richmond group. 

 The geographical distribution also is very extended, it having been found in Canada, New York, 

 Tennessee, Kentucky, Ohio, Indiana, Illinois, Iowa, Wisconsin, and Minnesota. 



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

 Museum Register, Nos. 6765, 7287, 7380. 



LIOSPIRA AMERICANA Billings. 

 (Not Figured.) 



Pleurotomaria americana BILLINGS, I860. Can. Nat. and Geol., vol. 5, p. 164. 



Pkurotomaria (or Baphistoma) lenticularis (part.) of American authors. (Not Sowerby's species.) 



Diameter 30 to 40 mm., bight one-half the diameter or less; apical angle about 130. 



This species grew to somewhat larger size than L. vitruvia, with which it is generally confounded 

 and excusably so, considering the imperfect condition of the great majority of specimens. We have 

 found it much less abundant than that species, while its vertical range also is less extensive, being 

 restricted apparently to the Trenton period. When good specimens are compared it may be distinguished 

 at once from L. vitruvia by the different shape of the under lip and a corresponding difference in the 

 course of the lines of growth. In L. vitruvia, namely, (see pi. LX1X, flg. 4) the central portion of the 

 lower lip projects greatly forward, the anterior outline being sharply rounded in consequence. In 

 L. americana, on the other band, the projection is much less and the curve of the outline, therefore, 

 broader, the conditions being about as in L. micula and L. progne. (See pi. LXVIII, flgs. 24 and 38.) 

 When the aperture is imperfect and the lines of growth are not preserved, we must depend upon the 

 characters of the umbilicus, which is a little wider and less abrupt in casts of L. americana, while the 

 shell itself presents no sign of the angulation which encloses the umbilicus in L. vitruvia. Where casts 

 of the interior only were available we found it sometimes impossible to distinguish them from L. progne, 

 but fortunately it is very rare to find casts of L. americana in which the umbilical cavity is freed 

 entirely of the matrix. This fact affords an almost infallible clue, for, if the cavity contains any of the 

 stony matrix in which the fossil was imbeded, the observer may rest assured that the specimen is not 

 one of L. progne. In the latter species the relatively large umbilical space is completely occupied by 

 shell-matter, so that the cavity remaining after the dissolution of the shell could not, under ordinary 

 circumstances, be filled with matter of the same kind as that which fills the interior of the whorls. 



Formation and locality. Stones River group, Lebanon, Tennessee; Black River group, Maury county, 

 Tennessee; Trenton group (Fusispira bed), Fillmore county, Minnesota; also several localities in 

 Manitoba. 



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

 Museum Register, Nos. 7348, ?7395. 



LIOSPIRA PROGNE Billings. 



PLATE LXVIII, FIGS. 3844. 



Pleurotomaria progne BILLINGS, 1860, Can. Nat. and Geol., vol. 5, p. 163. 



Width 25 to 35 mm.; hight about half the width; volutions four to four and a half; apical angle 

 about 120. 



This Is another of the forms that, especially where it occurs as casts of the interior, is generally 

 referred to as Raphistoma Unticulare =Liospira americana Billings sp. Although casts of the interior 



