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 Valley 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. 



