PELECYPODA. 151 



Hall's description — "Shell large; body ovate, acute, erect or moderately oblique; 

 wings more or less expanded, often greatly expanded; height frequently one-third 

 greater than the length; basal margin more or less regularly rounded; anterior side 

 broadly convex; posterior side nearly straight or slightly concave. 



Valves very unequal. Left valve varying from moderately convex to gibbous and 

 arcuate. Right valve depressed-convex in the umbonal region, flat or concave below; 

 variable in convexity. 



Hingeline straight, often greatly extended, equalling or usually greater than the 

 length of the valve. 



Beak of left valve prominent, acute, inclined forward, rising above the hinge- 

 line, and situated anterior to the middle of the body of the shell. Umbonal region 

 somewhat prominent, often gibbous, limited anteriorly by a distinct rounded sulcus, 

 and posteriorly by an interruption and change in the direction of the striae, which some- 

 times produces a defined line of separation. Umbonal angle acute. 



Anterior wing auriform and small in young specimens, becoming expanded and 

 variously extended in older specimens, limited by a distinct byssal depression; ex- 

 tremity rounded. Posterior wing large triangular; margin concave; extremity often 

 extended beyond the margin of the shell. 



Test thick, marked by fine, irregular, concentric striae of growth, which are some- 

 times raised into sharp lamellae, or crowded into prominent fascicles. These striae 

 become more conspicuous upon the margins and upon the wings, where they are often 

 highly lamellose. The distant lamellae which mark the body of the shell are sometimes 

 continued upon the hinge, and becoming stronger, curve over the margin, giving it a 

 sharply annulated aspect, which, when preserved in the impression of the hinge border, 

 might be mistaken for a more important organic marking. Partially exfoliated shells 

 show some obscure radiating lines which belong to the intimate shell-structure, and are 

 not external. 



Posterior muscular impression situated near the middle of the posterior slope, and 

 marked by irregular vascular striae; from the lower anterior side of this impression the 

 pallial line extends slightly downward, curving forward, and continuing in a line nearly 

 parallel to the margin it terminates in a small muscular impression within the rostral 

 cavity. The pallial line is frequently pitted or interrupted in its course, and from these 

 pits radiating grooves extend toward the margin of the shell. 



Ligamental area extending the entire length of the hinge, comparatively wide, 

 grooved longitudinally with fine, continuous parallel lines. Hinge with one or two 

 liryear, slightly oblique, lateral teeth on the posterior side, and with numerous erect or 

 oblique irregular folds along the cardinal line below the ligamental area, which terminate 

 on the inner margin in distinct crenulations. The cardinal wrinkles are smaller and more 

 regular anteriorly becoming large and irregular posterior to the beak." 



Remarks — Only two specimens have been collected and they are too imperfect 

 to warrant positive identification. 



Occurrence — St. Laurent limestone, 3 miles south of St. Marys. 



Genus Leptodesma Hall 



Leptodesma rogersi Hall 

 Plate 36, figure 5 



1884. Leptodesma Rogersi Hall, Pal. New York, vol. V, pt. 1, Lamellibranchiata, I, 



pp. 176-177, pi. 21, figs. 1-9. 

 1909. Leptodesma rogersi Grabau and Shimer, North American Index Fossils, I, p. 



426, fig. 556 b. 

 1913. Leptodesma rogersi Prosser and Kindle, Maryland Geol. Surv., Middle and Upper 



Devonian, pp. 254-255, pi. 29, figs. 7-10. 



