124 



narrowly rounded ears. Ventral valve concave, the curvature correspon- 

 ding to that of the dorsal valve. 



Area of dorsal valve lying in the plane of the lateral margia, 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 hinge-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. 



Surface with a set of fine rounded elevated radiating striae distant 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 

 strice; 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-hne towards the 

 centre of the shell, crossing each other. The specimens from the Trenton 

 limestone are usually without these undulations, 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-line, and in such cases the deflection takes 

 place at one fifth or one fourth the length from the beak. Occasionally a 

 broad rounded elevated mesial fold extended into a linguiform projection 

 of the middle of the front margin gives to the dorsal valve a trilobate 

 character. The area of the ventral valve is in general at right angles to 

 the plane of the lateral margins, but sometimes it slopes a httle forward. 

 In some the hinge-line is greatly extended, the cardinal extremities forming 

 projecting triangular cars. 



Width on hinge-line from one inch to one inch and a half Length variable, 

 from two thirds of the width to four fifths or a little more. 



Struiilwnwna deltoidea (Conrad) has the ventral valve convex and may 

 be always distinguished from this even when the hinge-line cannot be seen 

 by the small rounded umbo close to the Ix'ak. ;S'. camerata and ;S'. tenuis- 

 trinta (Conrad) have also the ventral valve convex. (See Plate 31 A, 

 Vol. I. Pal. N. Y.) 



Localiti/ and Fonnation. — Trenton limestone. City of Ottawa, rare ; 

 more common in the Hudson River group, Anticosti. 



OoUeetors. — E. Billings, J. Richardson. 



