442 Fossils of the Chazy Limestone. 



On comparing the Mingan Island specimens with small indi- 

 viduals of 0. tricenaria, I find that the hinge line of the latter is 

 proportionally a little shorter, while the dorsal valve has a scarce- 

 ly perceptible concavity. Large specimens of 0. tricenaria are 

 one inch wide at the hinge line, but the more common width is 

 nine lines. In the large individuals the beak is proportionally 

 more depressed than it is in the small ones. 



Locality and formation. — One specimen two miles north of 

 Montreal. More common at the Mingan Islands. Chazy lime- 

 stone. 



Collectors. — Sir W. E. Logan, J. Richardson, E. Billings. 



Orthis piger. (N. s.) 



Description. — Transversely semi-oval or sub-quadrate, hinge 

 line equal to the greatest width of the shell or nearly so; lateral 

 margins sub-parallel, straight or gently convex for a little more than 

 half the length from the cardinal angles ; front angles round- 

 ed, anterior margin straight or gently convex for half the width. 

 Length one fifth to one fourth less than the width. 



Ventral valve depressed conical, gently curved from the beak 

 for one fourth the length, then descending with a somewhat flat 

 slope to the anterior margin and cardinal angles. The beak in 

 the small specimens is the most elevated point, but in the larger, 

 the most prominent region is a little in front of the beak. Area 

 large, triangular, and gently arched upwards and outwards at an 

 angle of about 110° with the plane of the lateral margins. For- 

 amen not observed. 



Dorsal valve strongly and uniformly convex, with a moderate 

 compression at the cardinal angles ; area narrow, beak not very 

 strongly incurved. 



Surface with fine undivided radiating ridges of which there are 

 three in one line at the middle of the front margin in a specimen 

 seven lines wide, and four in a specimen five lines wide. These 

 ridges are crossed by fine closely arranged imbricating concentric 

 striae. 



Width of largest specimen seen, (a ventral valve) seven lines, 

 length from beak to middle of front margin six lines ; from hinge 

 line to front margin five lines; height of area two lines ; length 

 of hinge line six and a half lines ; greatest width at about one 

 third the length from the front. 



Width of small specimen five lines ; length of dorsal valve 

 four lines ; height of area of ventral valve one line and a half » 

 depth of both valves three lines. 



