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 strise appear upon some parts of 

 the surface of the specimen, which is a partial cast. k 



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 



