GASTEROPODA. 
79 
Pleurotomaria trilix. 
PLATE XXI, FIGS. 13-15. 
Pleurotomaria trilix. Hall. Descriptions of New Species of Fossils, etc., p. 17. 1S61. 
“ “ “ Fifteenth Rep. N. Y. State Cab. Nat. Hist., p. 45, pi. 5, fig. 1. 1862. 
“ “ “ Illustrations of Devonian Fossils: Gasteropoda, pi. 20. 1876. 
Shell turreted, subcorneal, higher than wide, consisting of four or more volu¬ 
tions, the apical ones of which are small, the last one ventricose, angular 
on the periphery, and concave below, with a distinct umbilicus. Aperture 
broadly suboval, higher than wide. 
Surface marked by a single sharp revolving carina between the suture and 
the peripheral band, and on the last volution by a similar carina, and 
sometimes a second less conspicuous one, below the band. The volutions 
are crossed by distinct, acute, concentric strife, which are usually distant 
more than their width, and sometimes closely arranged and little elevated. 
The peripheral band is triply carinate, inclosing two depressed spaces 
upon which the concentric strife are abruptly bent backwards from the 
aperture to the medial line on the upper side, and a little less abruptly 
forward below. 
This species, though observed only in imperfect specimens, is readily 
distinguished from any others of the group by its tricarinate peripheral 
band, and the single revolving carination above, and one, with a second 
subdued or obsolescent one below. 
The form of this species is less rotund than the preceding, but, from the 
crushed and distorted condition in which it usually occurs, it can be distin¬ 
guished only by the surface markings. The carinfe above indicated are less 
conspicuous than those of P. capillaria, and the strife are finer; and the well 
marked peripheral band with three revolving lines and with the carination 
below it in the line of the suture, in the higher volutions, is also a distinguish¬ 
ing feature. 
This species is far less numerous than the preceding form, and scarcely 
more than half a dozen specimens altogether have been observed. 
In the general aspect of its surface markings this species resembles P. 
