82 
PALAEONTOLOGY OF NEW YORK. 
bend gently backward. These transverse striae are cancellated by some¬ 
what finer 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 carinse, 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, 22. 
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 strife extend from the 
suture obliquely to the carina, and thence bend more abruptly backward 
to the peripheral band. Below the band the strife are directed forward 
to the first carina, and thence passing more directly over the lower side 
