9 IS THE PALEONTOLOGY OF MINNESOTA. 



tBellerophon platystoma,. 



Formation and locality. Black River limestone, Mercer county, Kentucky. 

 Collection. E. O. Ulrich. 



BELLEROPHON PLATYSTOMA Meek and Worthen. 



PLATE LX1V, FIGS. 22-30. 



Bellerophon (Bucania?) platystomi M. & W., 1868, Geol. Surv. 111., vol. iii, p. 312, pi. in, figs. 8a, 6. 



Shell exceeding medium size for the genus, composed of about three volutions, 

 which increase rather slowly in size until near the aperture where the last one is 

 suddenly and greatly expanded laterally; whorls embracing very little, subtri- 

 angular in cross-section, with the dorsum strongly carinate; surface descending on 

 each side from the keel, first with a concave then a nearly flat slope, toward the 

 edge of the umbilicus into which it turns very abruptly; umbilicus open, rather 

 large, about one-fourth as wide as the greatest diameter of the shell; aperture 

 somewhat triangular-reniform, the hight a little greater than half the width, the 

 width exceeding by nearly a fifth the greatest diameter of the shell; outer lip thin, 

 broadly sinuate, the center of bottom of sinus prolonged into a narrow slit; inner lip 

 apparently with but a very little developed callosity. Lines of growth sharp, rather 

 regular, curving backward gently between the umbilicus and carina. The latter, on 

 which we have not observed any well defined slit-band, is very prominent and 

 almost sharp on casts of the exterior, but on casts of the interior it is not 

 distinguishable from the general regularity of the dorsum. 



We are quite confident of the specific identity of the Minnesota shells above 

 described and the original types of B. platystoma, the latter having been examined 

 by us. In Minnesota we have two varieties of the species, one, agreeing exactly 

 with Meek and Worthen types, occurring in the Fusispira bed, the other, which is 

 much smaller, its greatest diameter but rarely exceeding 20 mm., being a common 

 fossil of the Clitambonites bed. 



B. platystoma is closely related to B. similis, but may be distinguished readily 

 enough by the different transverse section of its volutions, this being subtriangular 

 while in the new species it is semicircular rather than triangular. When, as is often 

 the case, the expanded aperture is broken away, the remaining whorls of B. 

 platystoma remind one greatly of Crytolites. Similarly imperfect examples of B. 

 similis, however, are scarcely distinguishable from B. troosti. None of the other 

 species known are closely related, nor have we experienced any difficulty in 

 separating B. platystoma from them. 



Formation and locality. The original types are from the Trenton (Galena) group at Galena and 

 IMxnti, Illinois. In Minnesota the small form is common in the Clitarnbonitos bed at localities in Good- 

 hue county, while the larger or typical form is not rare in the Fusispira bed at Kenyon, Holden P. O., 

 Wykoff, Weistiachs' dam, and other localites. 



