42 UNITED STATES GEOLOGICAL SURVEY OF.TBE TERRITORIES. 



Subgenus INOCERAMUS. 

 hioccranuis fragilis, H. & M 



Plate 5, fig. 5, and accompanying cuts. 

 Tnoccramua fragilis, Hall and Meek (1854), Meiu. Am. Acad. Arts and Sci. (n. s.), VIII, 388, pi. 2, fig. (1. 



Fig. 1. 



Fig. 2. 



Inoceramus fragilis, H. & M. 



1. View of viglit valve. 



2. Anterior profile view, showing the very nearly equal con 



vexity of the valves. 



Shell thin, broad-subovate, 

 higher than long, moderately con- 

 vex, subequivalve ; anterior side 

 vertically truncate from the beaks, 

 with a slightly concave outline ; 

 basal and posterior borders form- 

 ing a more or less regular, nearly 

 semicircular curve; hinge-line 

 rather short, and standing nearly 

 at right angles to the truncate 

 anterior. Beaks pointed, equal, 

 scarcely rising above the hinge, 

 curving inward and slightly for- 

 ward at the points. Surface 

 marked by fine lines of growth, 

 and a few obscure traces of concentric undulations. 

 Height, about 1.43 inches; length, 1 07 inches. 



The specimens from which the figure and description published by 

 Professor Hall and the writer were made out, being to some extent flattened 

 and distorted by pressure, as well as partly embedded in the matrix, its 

 characters could not be very clearly understood ; consequently, its truncate, 

 concave, anterior border was mistaken for the hinge-line, and placed upward 

 in the figure above cited. This misapprehension subsequently led to the 

 erroneous supposition that it was only a young individual of I. j'roblematicus, 

 Schloth. 



The specimens now before me, in part from the original locality, show 

 very clearly that the hinge is on the shorter side of the beaks, and that the 

 concave side is the anterior border; consequently, the form of the shell, is 

 quite different from that of I. problematicus or I. pseudo-mytiloides, at any 

 stages nl' their growth, being much more like that of /. substrkttus, Miinster, 



