GASTEROPODA . 
121 
Bellerophon nactus, n. sp. 
PLATE XXVI, FIGS. 17, 18. 
Shell below the medium size, ovoid, gibbous. Inner volutions small, the 
exterior portion of the last one regularly enlarging to near the aperture. 
Aperture moderately expanded; peristome auriculate at the postero-lateral 
margins, and partially curving around the open umbilicus. The dorsum 
with a sharp elevated carina. 
Surface marked by fine concentric striae, which in some parts are crowded in 
fascicles, curving gently over the sides of the shell and turned abruptly 
backward near the carina, in crossing which they make an acute retral 
angle. Obscure indications of revolving striae appear upon some parts of 
the surface of the specimen, which is a partial cast. 
This species is rare among the extensive collections made in this formation, 
and the few specimens observed are in a very unsatisfactory condition for 
illustration. The two examples figured, present much variation in form, the 
one having been compressed laterally, while the other, fig. 18, is of a larger 
size, obliquely compressed, and the surface striae are less conspicuous. 
Formation and locality. In the sandy shales of the Chemung group at Philips- 
burgh, N. Y., and on Keely creek, Tioga county, Pennsylvania. 
Of the twenty-two species of Bellerophon, described in the preceding pages 
all are of the typical form of the genus; the only deviation worthy of notice 
being in B. curvilineatus , which is a remarkably compressed form. In the New 
York formations of Devonian age, there is not a single species of the type which 
I have termed Bucania ; and the only one recorded as of this age is the B. 
Devonica, the relations of which may not have been satisfactorily determined. 
With this exception, so far as known to me, that genus is restricted to the 
Silurian and Cambrian formations of the country, a single species only being 
known in the Lower Helderberg group—the latest of these deposits. In the 
Niagara and Clinton groups six species are recorded, and another indicated 
16 
