6f, PALAEONTOLOGY OF NEW YORK. 



Plkukotomaria plena. 



plate xvii, fios. ii, 12, 13. 



Ptrurotomaria plena, Hail. Illustrations of Devonian FobsUb: Gasteropoda, pi. 17. 1876. 



Shell ovoid-conical, rotund ; spire elevated ; apex minute. Volutions five or 

 six, gradually expanding, rounded above, and somewhat abruptly curving 

 downward at the sides, the last one becoming very ventricose. The 

 periphery along the line of the band is apparently a little project- 

 ing beyond the regular curve of the volution ; lower side very convex 

 in the middle, and thence abruptly curving inward to the umbilical region. 



Surface marked by somewhat coarse, regular striae, about equal to the spaces 

 between, which are directed gently backward from the suture almost to 

 the limit of the peripheral band, being abruptly deflected at the junction. 

 The peripheral band is crossed by abruptly curving striae, and, on the 

 volutions preceding the last one, scarcely rises above the suture-line. No 

 revolving striae have been observed upon the body of the shell, though in 

 better preserved specimens they may exist. 



This species, in general form, differs from P. Lucina in that it is a more 

 erect shell, the spire is more elevated, and the volutions more gradually 

 expanding, while the latter presents a slight difference in its rotundity, and, 

 in the same way, a scarcely describable difference in the convexity of the 

 higher volutions ; it has also the two sets of striae almost equally conspicuous. 

 From the forms of P. arata known, P. plena differs in the greater elevation of 

 the spire, and the finer concentric striae, of which there are twice as many in 

 a given space. 



The shell is imperfectly preserved in the specimen figured, and the peripheral 

 band is much obscured, especially on the higher volutions. The peristome is 

 broken away to a considerable extent, and the form of the aperture can only 

 be inferred from a section of the volution, which is subcircular and slightly 

 transverse. 



Formation and locality. In limestone of the Upper Helderberg group, Helder- 

 berg mountains, Albany county, N. Y. 



