1( )2 PALJEOSTOLOGY OF SEW TORE. 



order, but as a rule they are arranged in lines corresponding to the arching 

 r.»ta' over which the callus lies. Tin.- i- especially marked in figure 8, plate 

 •J I. Ran example* are seen where the callus scarcely extends beyond the 

 thickened margin at the edge of the aperture, while the pustules are already 

 implanted upon the arching costte, and the spaces between them are gradually 

 becoming undefined or obsolete. The pustules are often elongate, extending 

 across two or more of the costae, as seen in figure 7, plate 24. These phases in 

 the extension of the callus arc shown in figures 27-29 of plate 22, and in figures 

 6-8 of plate 24. In a single longitudinal section the thickened columellar 

 lip is shown, as in figure 30 of plate 22, and more extremely on plate 20. 



In its general expanded form this species resembles the B. Leda and B. Lyra, 

 but differs in the external markings. It has also the same general form as 

 B. rudis of the Hamilton group, and B. trilirata of the Chemung group, but in 

 the former the strong distant costa? with distinct revolving stria? or ridges are 

 marked characters ; while the tricarinate aspect of the other, together with 

 the revolving stria? are distinguishing features. In its arching costse it closely 

 resembles the B. Newberryi; but in that species the volution is more depressed 

 on the back, with the callus of the columellar lip smooth, and extending 

 almost directly across the volution. 



Nearly all the specimens examined are more strongly striate on the back of 

 the volution, near the expansion, than the one figured in the Geological Report 

 of the Fourth District ; but in other respects there is no important difference. 



In the prevailing forms of this species the aperture measures, in the 

 transverse diameter, from one inch and five-eighths to one inch and six-eighths, 

 and in the longitudinal direction about one inch and a quarter. Larger 

 specimens have a transverse diameter of aperture of two and a quarter to two 

 and a half inches, with a dorso-vcntral diameter of about two to two and a 

 half inches. 



Formations and localities. In the coarse shales of the Hamilton group in 

 Schoharie county ; in Otsego, Onondaga and Chenango counties; on the shores 

 of Skaneateles, Cayuga, Seneca and Canandaigua lakes, and in the ravines 

 entering those lakes; more rarely at York in Livingston county; and in the soft 



