MUSEUM OF COMPARATIVE ZOOLOGY. 67 



less than diameter of shell ; both valves slightly, nearly equally convex. 

 Toothed valve a little the deepest. Transverse diameter a little greater 

 than from beak to border. Area of both valves narrow. Muscular im- 

 pressions of toothed valve broader than long, extending about one fifth the. 

 distance from beak to border. Surface with from eighteen to twenty-two 

 heavy, rounded ribs; the interspaces on the border double as wide as the 

 ridges. 



This species may be identical with 0. Jlubellulum var. Hall from New 

 York, but is certainly distinct from the 0. Jlubellulum Sow. — Salt Lake 

 Bay, upper part of Division E, Canada Geological Survey. 



Orthisina D'Orb. 



Orthisina diversa Soalkr. 



Svx. Orthisina Verneuilli Billings. 



Toothed valve usually pentagonal; socket-valve quadrate; hinge-line 

 usually equal to the greatest width of shell. Toothed valve very strongly 

 projecting; depth about one half the width; deepest point about the height 

 of hinge-line ; umbo somewhat laterally compressed, usually rising high 

 above the plane of the hinge-line, but very variable in this respect; umbo 

 always laterally inclined, indifferently towards either extremity of the hinge- 

 line. Surface near the extremities of the hinge-line a little depressed and 

 slightly recurved ; area very large, nearly half as wide as long. Fissure 

 from one fourth to one third the width of hinge-line ; deltidium large, 

 massive, rarely central, with distinct circular or oval foramen. Socket- 

 valve with a broad and shallow mesial fold. 



This form is closely related to 0. Verneuilli EiCil., but differs from it in 

 the size of the radial stria?, and in many important internal features. — Ellis 

 Bay, Anticosti. 



Platystrophia King. 

 Platystrophia regularis Shaler. 



Outline much the same as that of the other members of the group. 

 Socket-valve one fourth more projecting than the toothed valve: hinge-line 

 a little less than diameter of shell, three fifths greater than distance from 

 beak to border. That portion of the margin occupied by the fold and sinus 

 is re-entering, the depth of the incurvation being about equal to the eleva- 

 tion of the umbo above the hinge-line. The depression of the sinus is 

 occupied by only two plications, and the ridge by three similar folds. 

 On either side are from eight to nine plications. These numbers seem 

 invariable. 



The muscular impression of the toothed valve is long and narrow, length 

 being three or four times its width, extending nearly to the centre of the 



