138 



about the middle half of the whole length of the shell ; parallel to the 

 ventral margin nearly straight or slightly convex. The lower half of the 

 posterior margin is gently convex ; the upper half straighter and sloping 

 to the posterior end of the hinge line. 



From the beak a nearly straight fold runs down to the basal margin, 

 which it reaches at a point between the posterior fifth and sixth. Another 

 similar fold runs to the posterior margin a little above the mid-height. 

 Behind each of these folds the surface is slightly depressed, making two 

 step-like descents backwards. 



The dorsal margin, above the last fold, is compressed, slightly concave. 

 Just below the hinge line there is a row of, apparently twelve to fifteen 

 impressions of small teeth, which slope upwards and slightly backwards. 

 No teeth can be seen in front of the beaks. Surface of the cast, with 

 impressions of fine concentric striae, apparently six or seven in two lines. 



Length sixteen lines, height at the umbones ten fines. 



Variety. 



Plate 8, fig. 11. 



The above description is founded on the specimen represented by fig. 

 10. But there are several others, which differ from that in having the 

 posterior extremity obliquely sloping from the end of the hinge-line down- 

 wards (Fig. 11.) The beaks also vary in position from about the anterior 

 third to near the mid-length. At present, I shall retain them as a 

 variety. 



Locality and Formation. — Arisaig, Nova Scotia. Upper Silurian. 

 Collector,—!^, C. Weston. 



CyTHERODON ? SOCIALIS. (N. sp.) 

 Plate 8, fig. 12. 



Description. Transversely elongate ovate j length nearly thrice the 

 height ; moderately convex ; beaks between the anterior fifth and fourth ; 

 anterior extremity rounded, from three -fourths to seven-eighths the whole 

 height ; ventral margin gently convex ; hinge-line straight, a fittle more 

 than half the length between the beaks and the posterior end. A shal- 

 low groove runs from the beaks backwards and downwards, to the lower 

 posterior angle. The surface, as seen in the casts of the interior, is 

 slightly depressed above the groove. A second depression occupies a 

 small space below the hinge-line, and somewhat resembles a long lunette. 

 The posterior extremity is either doubly truncated or irregularly rounded. 



