MUSEUM OF COMPARATIVE ZOOLOGY. 65 



Leptoena Dalman. 

 Leptacna Julia Shaleb. 



Syn. Slrophomena Julia Billings. 



This species possesses all the important characters of the group, — sudden 

 deflection of the valves, corrugated surface, together with the ridge around 

 the visceral disk on the internal surface of the socket-valve. — Near South- 

 west Point, Anticosti. 



Leptaena quadrilatera Shaler. 



Syn. Slrophomena depressa Billings. 



Shell margin broadly semi-oval ; greatest width at hinge-line, which is 

 one half longer than from beak to border. Tooth-valve with the visceral 

 disk nearly rectangular; laterally about two sevenths wider than from beak 

 to point of deflection; disk convex near the umbo, concave near the deflected 

 margin; umbo rather prominent, broadly rounded, scarcely rising above the 

 hinge-line ; radii fine, with even interspaces, somewhat irregular on the de- 

 flected margin ; concentric undulations about six in number, wanting on a 

 space about one fifth of an inch wide near the umbo ; hinge area very nar- 

 row, almost linear near the extremities; socket-valve nearly flat or slightly 

 convex over the visceral disk, usually with a distinct mesial sinus; hinge 

 area narrow, not over one twenty-eighth of an inch in width; cardinal 

 process minute. 



This species may prove identical with the form from the Niagara group 

 of New York, but there are several constant exterior differences. The 

 interiors have not been examined. — Ellis Bay, Anticosti. 



Orthis Dalman. 



Orthis laurentina Billings, Report Can. G. S. for 1857, p. 297. — Junction 

 Cliff, Anticosti. 



Orthis media Shaler. 



Syn. Orthis elegantula Bixlings. 



Shell orbicular ; hinge-line one half less than width of shell. Toothed 

 valve evenly convex ; depth in adult specimens about one fourth the height, 

 in young specimens proportionately a little greater; umbo slightly elevated, 

 rising above the hinge-line one eighth the distance from beak to border, 

 slightly compressed, occupying at the hinge-line about one fourth the 

 diameter of the valve ; beak small, distinct, slightly recurved, a little 

 overhanging the area ; area small, rather broad. Fissure triangular, one 

 third as wide as length of hinge-line. Socket-valve transversely flattened, 

 a slight mesial depression dividing the surface into two lobations. 



Differs from its European representatives, being more orbicular, having 

 a less projecting umbo, less incurvation of beak, much finer radial strife, 

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