Fossils of the Chazy Limestone, 



435 



Safford, State Geologist of Tenne c see, has sent me some very gib- 

 bous forms of 0. perveta, and they very much resemble O.gibbosa. 

 It may be that, by comparison with extensive series of western 

 specimens, these three species might be united. I shall for the 

 present keep them separate provisionally. 0. gibbosa occurs 

 rarely in the Chazy limestone, island of Montreal ; abundantly, 

 but badly preserved, at the Pallideau Islands, Lake Huron, in 

 rocks which are either Chazy or Black River, At La Petite 

 Chaudiere rapids near Ottawa, and at the Fourth Chute of the 

 Bonne-chere, in the Black River limestone, and in the Trenton 

 limestone at Belleville. The other two species I have seen in the 

 Chazy only, but in the Western States they are known in higher 

 rocks. 



Orthis impbrator, (N. s.) 



Fig. 11, 





•%^s 



HtUjjii 1 1 l->&£#iU*iik« K 



Fig. 13. 



Description. — Subquadrate, laro-e, very gibbous, hinge-line less 

 than the greatest width of the shell ; front margin gently convex, 

 straight or slightly concave; front angles rounded ; a portion of 

 the sides, equal to about one third the length along the middle, 

 usually straight, but sometimes a little convex, above which the 

 sides curve inward to the cardinal angles. 



The ventral valve is moderately and somewhat irregularly con- 

 vex, the beak small, pointed, and much elevated. In most of the 

 specimens a broad, low, mesial riclge or depressed fold extends 



