176 
PALAEONTOLOGY OF NEW YORK. 
Test thin, marked by fine, closely arranged, concentric striae, which are 
more crowded anteriorly; these are crossed by extremely fine radii, which 
are chiefly confined to the shallow sulcus, anterior to the beak, and are also 
obscurely marked on the wing. 
Ligamental area marked by two strong grooves. Two oblique linear 
depressions along the posterior slope indicate the probable existence of lat¬ 
eral teeth. Other characters of the interior, unknown. 
The specimen has a length of 11 mm., height 7 mm., hinge-line 12 mm. 
This species bears some resemblance to several species in the Chemung 
group, but it is comparatively narrower and more oblique. 
Formation and locality. In the Marcellus shale, Bloomfield, Ontario county, 
N. Y. 
Leptodesma Rogersi. 
PLATE XXI, FIGS. 1-9. 
Leptodesma Rogersi. Hall. Pal. N. Y., vol. v, pt. 1. Plates and Explanations : PI. 21, figs. 1-9. Jan., 1883. 
Shell of small or medium size, sub-rhomboidal; body ovate, very oblique; 
length greater than the height; anterior and basal margins broadly rounded; 
posterior margin extended and abruptly recurved. 
Valves equally convex above. Right valve somewhat depressed below, 
comparatively higher than the left. 
Hinge-line straight, longer than the length of the shell. 
Beaks sub-anterior, obtuse, nearly erect, prominent. Umbonal region 
gibbous, oblique. 
The anterior extremity is scarcely alate or auriculate, consisting of a 
rounded extension, straight above and slightly sinuate at the base. Wing 
comparatively large, triangular, joining the body of the valve near the poste¬ 
rior extremity, defined by the crowding and curving of the concentric striae; 
margin nearly straight for five-sixths of its extent, then acutely recurving; 
extremity prolonged into a mucronate spine which extends beyond the poste¬ 
rior limit of the valve. In the right valve the wing is less deeply sinuate. 
