OF DENISOX UNIVERSITY 6 1 



obliquely ovate, length considerably exceeding the height ; post basal 

 side considerably extended ; ear straight, almost acute, anterior side 

 obliquely truncate; body making an angle of about 45°, with the 

 hinge line. Left valve shortest, most gibbous: beak one fourth from 

 anterior, rather prominent, but not incurved, ear rather convex, with 

 a broad shallow byssal sinus, wing large flat, the posterior margin 

 meeting the hinge at an angle of about 90° or more, acute at the very 

 angle only. The hinge margin is greatly thichened. The difference 

 between the two valves is slight, the beak of the right being generally 

 in advance of the other, and the front margin is more uniformly curved. 

 Eniire length of hinge linge 22 mm, height 14 mm, length from 

 beak to antero- ventral angle 19 mm. Surface marked by concentric 

 lines. This species somewhat resembles L. rafinesquii, H , but differs 

 in being more oblique and longer, with straighter and longer hinge and 

 less produced beak and straighter posterior margin. Leptodesma hec- 

 tor is somewhat more like our species, but exhibits an opposite extreme. 

 This species is characteristic of a zone several feet thick, about 40 feet 

 below conglomerate I, in connection with other fossils of a more or 

 less Devonian habit. This species may be compared with L. torreyi, 

 but is not so produced: it also resembles Leptodesma nereus. Compare 

 also L. naviforme. 



Leiopieria halli, ^p- "• 

 (Plates VII, Fig. 31, Cf Fig. 32.) 



Shell of moderate size ; sub-rhomboidal ; body narrowed above, 

 obliquely ovale; height greater than length at hinge; margin strongly 

 sinuous; ears rounded, somewhat depressed, short ; wings broad, e.x- 

 tended, acute : hinge-line straight. Valves nearly similar, left valve 

 rather more convex. Right valve moderately convex ; body diverging 

 from hinge-line at an angle of about 45° ; wing acute ; posterior mar- 

 gin strongly concave, postero-ventral extremity rather sharply curved ; 

 anterior margin abruj)tly flexed near the middle, anterior to which it is 

 gently concave; ear rounded, dei)ressed. Left valve with smaller 

 wing more convex, especially ventrally. Surface of both valves 

 marked by conspicuous concentric folds. Hinge with a decided 

 groove and thickened ridge. Beak of right valve high, but not pro- 

 jecting dorsally, that of the left valve extending beyond hinge-line. 



