438 Fossils of the Chazy Limestone. 



is broadly convex, except near the beak, where it is narrowly gib- 

 bous and suddenly depressed, to the flat or concave space at the 

 cardinal angles. 



In many respects this species resembles 0. imperator, but is 

 never more than one-fourth the bulk of that species, and differs 

 from it constantly by the sinus in the front half of the ventral valve. 

 It also occurs in the greatest profusion in localities where the 

 other is not found at all, while in other places the two are asso- 

 ciated. 



It is closely allied to 0. pectinella, but is always much more 

 convex and more numerously ribbed. 



The width is usually nine lines, length about seven lines ; 

 depth of both valves in a very gibbous specimen, six lines. 



Locality and Formation. — Caughnawaga ; St. Genevieve ; Isle 

 Bizard ; near the village of St. Laurent, and near Cornwall. At 

 Caughnawaga it occurs in vast abundance, but rarely perfect. 

 Chazy limestone. 



We have several specimens from the Trenton limestone at 

 Ottawa and Belleville which seem to be of this species. 



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



Bell. 



Orthis platys, (N. s.) 



Fig. 15. 



Fig. 15. — Orthis platys. a, dorsal aspect; b, longitudinal section ; 

 c, ventral valve. 



Description. — Semi-oval, or sub-quadrate, both valves nearly flat ; 

 sides straight and sub-parallel for one half the length from the car- 

 dinal angles; front broadly rounded ; hinge-line straight, equal to 

 the greatest width of the shell ; width one third greater than 

 the length. 



Ventral valve with the beak slightly elevated, depressed convex 

 near the beak, gently concave all round near the margin ; a broad, 

 very shallow mesial depression extending from the front margin 

 to about the centre of the shell ; area moderate, forming an angle 

 c fabo t 115° with the plane of the margin ; foramen triangular, 

 extending to the beak, which is small-pointed and a very little 

 depressed, not incurved at the point. 



