918 THE PALEONTOLOGY OF MINNESOTA. 
[Bellerophon platystoma. 
Formation and locality—Black River limestone, Mercer county, Kentucky. 
Collection.—E. O. Ulrich. 
BELLEROPHON PLATYSTOMA Meek and Worthen. 
PLATE LXIV, FIGS, 22—30. 
Bellerophon (Bucania?) platystoma M. & W., 1868, Geol. Surv. Ill., vol. iii, p. 312, pl. m1, figs. 8a, b. 
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 
Dixon, Illinois. In Minnesota the small form is common in the Clitambonites 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, Weisbachs’ dam, and other localites. 
