74 



Dorsal valve gently concave ; area slightly inclining forwards, forming 

 with the area of the other valve an angle of about 100°. 



Foramen of ventral valve about as wide as it is high, the apical third 

 closed by a thin deltidium. Area of the dorsal valve crossed by a groove 

 as in L. decipiens. 



The surface of this species appears to be smooth, but as all the specimens 

 are silicified it is possible that this may be the result of some circumstance 

 in the process of fossilhzation. 



Width about 3 Imes, length 2J lines. Many of the specimens are a 

 little larger. 



Locality and Formation. — Point Ldvis. In the upper part of the 

 limestone No. 2, Quebec gi-oup. 



Collectors. — Sir W. E. Logan, J. Richardson, R. Bell. 



Lept^na decipiens. (N. sp.) 



Fig. 67. a, b, c. 



9 



Descrij)tion. — Shell rather small, about the size of i. sericea, semi-oval, 

 widest on the hinge-line, usually narrowed from the cardinal angles forward ; 

 front margin rounded; length from two-thirds to three-fourths of the width. 

 Ventral valve convex, somewhat flattened in the upper half; umbo narrow 

 and slightly elevated, the convexity extending therefrom gradually widening 

 towards the front ; the cardinal angles compressed and sometimes slightly 

 reflected ; front half curved towards the dorsal valve with a rounded slope. 

 The area is rather large, somewhat concave, forming an obtuse angle with 

 the plane of the lateral margin. Foramen triangular, usually open, but 

 sometimes with a rudimentary deltidium over a small part at the beak. 



Dorsal valve concave, with a distinct mesial sinus commencing at the 

 beak and gradually widening forward, becoming obsolete before reaching 

 the front margm. Area well developed, flat, rather more than one-third 

 the width of that of the ventral valve, forming a right angle with the 

 plane of the lateral margin. 



Interior of ventral valve with a well-defined ridge running all round, 

 parallel with and at about one-third the length of the shell from the margin. 

 The dental plates are rudimentary, and seem to converge towards each 

 other, thus exhibitmg a tendency to form a small chamber in front of the 

 beak. 



Interior of dorsal valve with a semicircular ridge similar to that of the 

 ventral valve, but apparently a little nearer the margin. In some speci- 

 mens a mesial ridge extends from near the foramen towards the front, termi- 



