436 Fossils of the Chazy Limestone, 



from the beak to the front margin ; sometimes this fold is barely 

 perceptible or obsolete, and in such cases the whole of the valve, 

 except a small space at the cardinal angles, is flat, and slopes with 

 scarcely any curvature from the beak to the front ; at each of the 

 cardinal angles a portion of the shell is depressed towards the 

 dorsal valve. Area large, triangular, a little arched. Foramen 

 not large, extending nearly to the apex of the beak. 



Dorsal valve very convex, most elevated in the upper half, and 

 sloping abruptly to the sides, front and cardinal angles, the latter 

 strongly reflected, as is also in some specimens a narrow border 

 along the upper one third of the sides ; the beak and area strongly 

 incurved over the hinge-line. Along the middle of this valve a 

 broad, shallow, mesial furrow extends from the beak to the front. 

 The foramen is occupied by a sharp cardinal process. 



The surface is covered with moderately coarse, radiating ridges, 

 about four or five in the width of two lines, at the margin. They 

 appear to be two or three times subdivided between the beak and 

 the front. 



Width from one inch to one inch and a half; length about one 

 sixth or one fifth less than the width. 



This magnificent Orthis attains the size of the largest specimens 

 of 0. occidentalism and somewhat resembles that species, but is 

 always easily distinguished therefrom by the position of the mesial 

 sinus, which occupies the dorsal valve instead of the ventral. 



Locality and Formation. — Near Cornwall and in the township 

 of East Hawkesbury. Large loose blocks of the rock several feet 

 in diameter are occasionally met with, which are entirely com- 

 posed of imperfect shells of this species, with only a small admix- 

 ture of others. Judging from the appearance of these boulders, 

 it must occur in vast abundance in the beds from which they were 

 derived. It has been found in beds in situ only in Hawkesbury. 

 Chazy limestone. 



Collectors. — A. Murray, J. Richardson, R. Bell. 



Orthis borealis, (N. s.) 



Description. — Transversely broad oval or sub-quadrate ; width 

 usually one fifth or one fourth greater than the length ; both 

 valves more or less convex, sometimes very gibbous. 



Ventral valve rather convex ; greatest elevation at about one 

 fifth or a little less from the beak, thence sloping somewhat ab- 

 ruptly to the cardinal angles and more gently to the sides and 



