Fossils of the Chazy Limestone, 44t 



rounded undivided ribs. Width at the hinge line a little greater 

 than the length. 



The ventral valve of this species is strongly elevated with a 

 large area inclining upwards at an angle of a little more than 

 100,° and slightly arched. From the pointed beak the outline 

 makes a nearly regular curve to the front. Some of the speci- 

 mens have along the middle, from the beak to the front, a broad 

 carination on each side of which the surface slopes with a gentle 

 or flattened curve to the lateral margins ; others are uniformly 

 convex. The area is large, triangular, and with a very narrow 

 foramen which extends quite to the beak. 



Dorsal valve nearly flat, a shallow mesial depression along the 

 middle from the beak to the front. On each side of this depres- 

 sion there is a very slight convexity, and then a flat slope to each 

 of the cardinal angles. The area is narrow and sloping upwards 

 and outwards at an angle of a little more than 100° with the 

 plane of the lateral margin. It is divided in the middle by a 

 small triangular foramen. 



Width at hin<xe line four lines : length of dorsal valve three 

 lines; length of ventral valve from beak to front, three and a 

 half lines ; height of area of ventral valve one line and one third. 



The specimen from which the above description is drawn is 

 the only one, except those procured at the Mingan Islands, that 

 I have ever seen in the Chazy limestone. In its form and dimen- 

 sions it agrees so nearly with the descriptions of Conrad and 

 Hall, and with the figures in the Pakeontology of New York^ 

 that there can be scarcely any doubt of its being the same 

 species. 



At the Mingan Islands a number of good specimens of an 

 Or (his have been collected which are precisely the same as the 

 one above described in every particular, except that they are 

 upon an average two thirds larger, and in some of them the beak 

 is not proportionately so much elevated. In others the beak is 

 quite as prominent as it is in the Montreal specimen, and I do not 

 think it possible, therefore, that they can constitute a distinct 

 species. 



Orthis costalis (Hall) has never been sufficiently described and 

 illustrated to enable us to recoirnize it with certaintv. It i^ said 

 to have a flat dorsal valve and about 32 ribs, but the figures shew 

 from about 32 to less than 20. It may be that when better spec- 

 imens are procured, it will appear that 0. costalis and O* dispa- 

 ralis are the same. 



