64 BULLETIN OF TIIE 



striae, two or three of which become larger than the others as they approach 

 the border. The finest striae are scarcely visible to the naked eye. — Jump- 

 er's Cliffs, near Southwest Point, Anticosti. 



Plectambonites Pander. 



Pleetambonites glabra Shaler. 



Syn. Leptcena sericea Billings, Can. Geol. Survey, 1853 -5G, p. 252. 



Shell elongate semi-oval ; distance from beak to border about one half as 

 long as hinge-line; hinge-line usually equal to the greatest width of shell; 

 outline evenly rounded. Toothed valve strongly convex; depth about two 

 fifths (sometimes one half) the length from beak to border; most convex 

 point one third the distance from beak to border; a little flattened near 

 the lateral border; strongly ventricose in the middle; umbo vising above 

 the hinge-line; slightly incurved; beak not distinct; area narrow ; in the 

 same plane with the lateral margin ; teeth small, slightly projecting. Socket- 

 valve curved to fit the toothed valve ; area a little less in width. 



Surface of valves, with very numerous nearly microscopic radial stria?, 

 closely set, of nearly equal size, scarcely distinguishable upon the centre, 

 but distinct upon the borders of the shell. The socket-valve has distinct 

 radial striae of a larger size interspersed among these, like the radii on the 

 toothed valve. 



This form differs from its representatives of the Lower Silurian by its 

 great convexity, as well as by many other characters. — Ellis Bay, Anticosti. 



Plectambonites area Shaler. 



Syn. Leptcena transversalis Billings. 



Shell semi-elliptical ; greatest width at hinge-line ; hinge-line one third 

 longer than from beak to border. Toothed valve in adult specimens very 

 ventricose; depth equal to one third the length of hinge-line; area narrow, 

 almost linear; fissure very small, equilaterally triangular; muscular im- 

 pressions indistinct. Socket-valve concave, flattened towards the ex- 

 tremities of the hinge-line; area narrow, linear; cardinal process very 

 small ; muscular impressions very slight. Surface of shell with twelve to 

 twenty distinct radii, with very fine radii between. — Near Southwest 

 Point, Anticosti. 



Plectambonites tenera Shaler. 



Very closely allied to the form called by Hall Leptcena iransversalis, 

 from the Niagara Lime of New York; it is, however, less convex, and more 

 flattened towards the extremities of the hinge-line. The area of the toothed 

 valve is in the same plane as the margin. The umbo scarcely extends above 

 the hinge-line in many of the specimens. Interior has not been compared. 

 — Near Southwest Point, Anticosti. 



