58 Silurian Fossils of Canada, 



narrowly rounded ears. Ventral valve concave, the curvature 

 corresponding to that of the dorsal valve. 



Area of dorsal valve lying in the plane of the lateral margin, 

 about one third of a line high. Area of ventral valve forming a 

 right angle with the marginal plane, in large specimens one line 

 or a little more in height at the beak, and gradually decreasing 

 towards the extremities of the hino-e-line. 



Foramen of ventral valve triangular ; the width at the base 

 somewhat exceeding the height, completely closed by a convex 

 deltidium, the basal margin of which is rendered a little concave 

 by the convex margin of the similar deltidium which closes the 

 foramen of the dorsal valve. 



Pig. 6.-—Strophom.ena fluctuosa. a, section, the dotted line represents 

 the plane of the lateral margin, and it is drawn to shew that 

 the area of the concave or ventral valve is at a right angle 

 to it. 



Surface with a set of fine rounded elevated radiating striae dis- 

 tant from each other usually about half a line, sometimes a little 

 less and occasionally one line. Between each two of these there 

 are from two to ten much finer striae ; the whole crossed by fine 

 crowded concentric lines. In most of the specimens the whole of 

 the upper half of the shell is covered with short undulating 

 wrinkles, which sometimes have a concentric arrangement and 

 often form concentric rows converging from the hinge-line to- 

 wards the centre of the shell, crossing each other. The speci- 

 mens from the Trenton limestone are usually without these undu- 

 lations, but in those from the Hudson River group this character 

 is prominently exhibited. 



This shell is somewhat variable in its characters. The visceral 

 disc of the dorsal valve is sometimes confined to a small area 

 around and in front of the beak and along the hinge-hne, and in 

 such cases the deflection takes place at one fifth or one fourth the 

 length from the beak. Occasionally a broad rounded elevated 



