82 PALEONTOLOGY OF NEW YORK. 



bend gently backward. These transverse stria3 are cancellated by some- 

 what liner revolving ones which mark the entire surface, except the 

 peripheral band, and notably the narrow depressed space between this 

 band and the lower carina. The peripheral band is narrow, strongly 

 limited on the two sides by slender carina?, and crossed by fine curving 

 striae. 



In the upper volutions the lower carina is a little above the suture-line, in 

 which respect it differs conspicuously from P. trilix or P. capillaria. It differs 

 from these also in the proportionally greater elevation of the spire, and the 

 less expansion of the last volution, as well as in the numerous fine revolving 

 striae. 



Formation and locality. In the shales of the Hamilton group, at Monteith's 

 Point, Canandaigua lake, N. Y. 



Pleurotomaria planidorsalis. 



plate xxi, figs. 21, 11. 



Pleurotomaria planidorsalis, Hall. Illustrations of Devonian Fossils : Gasteropoda, pi. 20, figs. 28, 29. 1876. 



Shell depressed-trochiform, subdiscoidal ; spire moderately elevated. Volu- 

 tions (entire number unknown) subangular on the upper side, rounded 

 below, the last one ventricose towards the aperture, which is transverse, 

 subrhomboidal, straight upon the upper side, and somewhat rounded 

 below. 



Surface on the upper side of the volutions marked by a strong revolving 

 carina, which is about twice as far from the suture as from the peripheral 

 band ; upon the lower side, at about the same distance from the periphery, 

 is a similar carina, and at about one-third the distance between this and 

 the umbilicus there is another" less conspicuous ridge which becomes 

 obsolete towards the aperture. Strong concentric striae extend from the 

 suture obliquely to the carina, and thence bend more abruptly backward 

 to the peripheral band. Below the band the striae are directed forward 

 to the first carina, and thence passing more directly over the lower side 



