PTEROPODA. 
179 
where this line meets the aperture it often produces a slight notch in the mar¬ 
gin, giving the characters suggestive of the original name. When occurring in 
calcareous bands this feature is not characteristic. 
This little fossil presents such a variety of aspect that I had already desig¬ 
nated the various prevailing forms as distinct species, before having had the 
opportunity of examining the larger collection from which many of the illus¬ 
trations on plate XXXI A are drawn. A final comparison among a very 
large number of specimens seems to indicate that these phases are due to the 
presence or absence of certain external features originally characterizing the 
individual, which have been modified to a considerable degree by the nature of 
the sediments and the conditions of preservation. 
Formations, localities, etc. The most easterly appearance of this species is in 
the black Marcellus shale at the base of the Hamilton group, in Schoharie 
county; at Cherry Yalley and other localities in Otsego county. It is found 
near Alden, Erie county, where it is extremely abundant, the shale being com¬ 
pletely filled with this fossil and Tentaculites gracilistriatus, and its occurrence is 
likewise known quite to the western limits of the State. In the shales of the 
Hamilton group, it is associated with a stouter form. In the Genesee slate it is 
extremely abundant, covering large surfaces, and forming alone, or with the 
Tentaculites, calcareous bands several inches in thickness, the whole having a 
subcrystalline aspect. This feature has been especially noticed in the upper 
part of the slate in Bristol, Ontario county, and also in Erie county. It occurs 
at Ithaca, and in the Cashaqua shales, near Mount Morris in the Genesee val¬ 
ley, and elsewhere in central New York. In the green shales of the Portage 
group it is common, but less abundant than in the Genesee slate. In the latter, 
throughout central and western New York, this species is associated with 
Chonetes lepida, and in the black slate at Lexington, Indiana, we find the same 
association of species. 
At an exposure of the Genesee slate, on the Cayuga creek, two and a half 
miles southwest of Alden, in Erie county, there is a calcareous layer of six 
inches thick, composed entirely of the shells of Styliola fissurella, with a small 
number of Tentaculites gracilistriata. 
