54 



is elongate, ovate ; rounded at the apex; the sides most prominent a little^ 

 above the middle j thence narrowing to a point at the base. The 

 posterior side of the cast has a compressed margin, about two lines 

 wide at the end of the hinge-line, gradually becoming obsolete towards 

 the base. On the anterior side, beneath the beaks, there is a concave 

 space which dies out before reaching the middle. About one line below 

 the beaks there is an obscure projection, indicating the anterior extremity 

 of the hinge-line. 



The surface shows a few obscure concentric lines and faint radiating 

 grooves. These latter, although apparent on the cast, may not occur on 

 the surface of the perfect shell. 



This species differs from M. Canadensis in having only a small portion 

 beneath the beaks depressed, while that one has nearly the whole of the 

 anterior side concave. Height of the largest specimen collected, twenty- 

 one lines; width a little below the middle, thirteen lines; depth of both, 

 valves, twelve lines ; length of hinge-line, six lines. 



Locality and Formation. — Indian Cove ; Gaspe limestone, No. 8,. 



Collector.—R. Bell. 



Leptodomus Canadensis. (N. sp.) 



Plate 5, fig. 1. 



Description. — Transversely elongate, oblong. Dorsal margin, as 

 seen in the cast of the interior, nearly straight, gently concave, slightly 

 the most elevated at the umbones. Ventral margin gently convex ; a 

 small portion at the anterior third concave ; the posterior third ascending 

 with a moderately convex curve. Anterior extremity slightly sinuated 

 at about the mid-height; below the sinus, a little projecting and narrowly 

 rounded ; nearly vertical above ; rounded over the umbones. Posterior 

 extremity with the most projecting point just above the mid-heigjit ; 

 gently rounded below ; obliquely truncated above. 



The shell is rather strongly convex, most prominent about the middle 

 or a little in front thereof. The umbones are large, beaks incurved. 

 From the anterior third of the ventral margin an obscure shallow^ depres- 

 sion in the surface ascends obliquely upwards and forwards, dying out 

 on the umbo, before reaching the beak. The umbones are sub-carinate on 

 their upper and posterior edge. The dorsal margin is inflected, with 

 some indications of a long narrow lunule. 



Surface, as appears by the cast of the interior, with strong concentric 

 undulations, three or four lines wide towards the posterior ; narrower and 

 deeper on the umbones. 



