GASTEROPODA. 79 



Pleurotomaria trilix. 



PLATE XXI, FIGS. 13-15. 



Pleurotomaria. trilix, Hall. Descriptions of New Species of Fossils, etc., p. 17. 1861 



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 striae, 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 striae 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 carinas above indicated are less 

 conspicuous than those of P. capillaria, and the striae 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. 



