Fossils of the Chazy Limestone. 



437 



front angles ; the front margin more or less depressed by a broad* 

 concave, mesial sinus of variable depth, which extends about half 

 the length or a little more towards the beak, and there gradually 

 disappears. Cardinal area large, slightly curved, and sloping 

 upwards at an angle of 115° with the plane of the margin. 



Dorsal valve convex, the amount of the convexity varying 

 greatly; cardinal angles compressed and reflected. Area narrow, 

 in the same plane with the margin ; umbo in the very convex 

 individuals large, and projecting beyond the hinge-line. 



Surface covered with strong, rounded, radiating ridges, which 

 in general extend without subdivision quite to the beak, those near 

 the cardinal angles much smaller than those in the central and 

 front region. There are four or five ridges in two lines at the 

 front margin, and ten or twelve in the first two lines of the lateral 

 margin, below the cardinal angles. 



Fig. 14. 



Fig. 14. — Ortkis borealis. a, rentral aspect ; 6, side view ; c, dorsal 

 aspect. 



Specimens of this species with the surface well preserved are 

 difficult to procure, and I think it highly probable that in many 

 the radiating ridges are subdivided or increase by implantation ; 

 but in the only perfect ones I have seen they are simple quite to 

 the beak. In a specimen nine lines in width there are fifty-one 

 ribs, including the finer ones at the cardinal angles, but the aver- 

 age number is between forty and fifty. 



This species is somewhat variable in form. Some of the spe- 

 cimens are sub-quadrate, with the cardinal and front angles 

 rounded, the sides being nearly straight, and in these the greatest 

 width is usually near the front. Others have the sides convex, 

 and these have the greatest width about the middle. Often the 

 dorsal valve has an indistinct mesial fold extending from the beak 

 to the front, while others have a slight depression, the effect of 

 which, together with the depression in the ventral valve, is to 

 produce a gentle sinuosity in the anterior margin. In general 

 however, the dorsal valve has neither a mesial fold or sinus, but 



