L . I MEL L IBRANCHIA TA. 1 1 1 7 



Ear short, limited by an undefined sulcus and shallow byssal sinus. Wing 

 large, triangular, nearly Hat, defined by the retral curving of the lamellose 

 stria 1 : margin moderately concave; extremity abruptly acute. 



Test thin, marked by regular concentric lines of growth, with distant, 

 lamellose striae, which are raised into thin, sharp elevations corresponding 

 with the concentric folds or undulations of the shell. The cast shows only 

 the undulations. On the anterior margin and wing, the lamellae are closelj 

 crowded, producing a varicose appearance. They are more closely arranged 

 upon the wing than on the valve, making a gentle retral curve and becoming 

 very conspicuous on the hinge-margin. 



Interior unknown. Ligamental area narrow. 



A left valve has a length of 4ii mm., height 49 nun., hinge-line about 

 47 mm. 



This species, in general proportions, resembles A. Bigsbyi ; it differs in the 

 larger and more extended posterior end, with stria* nearly vertical or gently curv 

 ing, except at the hinge margin; the body wider above, and the anterior basal 

 margin less convex. With a single exception, the specimens of L. Bigsbyi 

 occur in arenaceous beds, while this species is common both to the softer shales 

 and in the coarser beds. 



Formation and localities. In soft shales of the Hamilton group, from loose 

 masses of rock at the south end of Seneca lake, and in the coarse grits, at 

 Schoharie, N. Y. 



Leiopteria Troosti, n. sp. 



PLATE LXXXV1II, PIGS 12, IS. 



Shell above the medium size, suh-rhomboidal ; both' broadly ovate, moderately 

 oblique : height somewhat greater than the length ; anterior margin, from 

 the wing to about half the height, nearly vertical, curving broadly around the 

 base, and more abruptly rounded behind. 



Left valve moderately convex, gibbous on the umbo. Right valve unkni »wn. 



Hinge-line straight, somewhat less than the length of the shell. 



