LAMELLIBRANCHIATA. 21] 



LEPTODESMA TRUNCATUM, 11. s]>. 

 PLATE \( . in.- 34, i - .. 



Shell above the medium size, sub-rhomboidal : body ovate-elongate, oblique to 

 the hinge-line at an angle of about 35°: length two-thirds greater than the 

 height: ante-byssal margin oblique and often abruptly truncate, curving into 

 the sinus; broadly rounded along the ventral margin, and more rapidly curv- 

 ing on the post-basal margin. 



Left valve gently convex below, becoming more convex above, and some- 

 what gibbous on the umbo. Right valve less convex. Hat in the lower part. 



Hinge-line straight, nearly as long as the length of the shell. 



Beaks sub-anterior, directed forward : in the left valve somewhat promi- 

 nent; in the right valve not elevated above the hinge-line. Umbonal angle 

 acute. 



Anterior end obtuse, truncate, abruptly angular at the cardinal margin, 

 limited by a slightly oblique byssal depression. Wing large, triangular, 

 joining the body at the posterior extremity : margin sloping gently forward 

 for two-thirds of the width, then abruptly recurved and extended in an acute 

 or spiniform process. In the right valve the wing is less defined, and the 

 anterior end is more strongly truncate. 



Test thin, marked by sharp, elevated, distant, concentric stria-, with 

 intermediate finer lines of growth. The stronger stria* are regular upon the 

 wing and posterior part of the shell, fasciculate on the anterior side, ami 

 stronger and lamellose on the anterior extremity. 



Interior unknown. 



A left valve has a length of 38 mm., height 'I'l mm., and hinge-line 37 mm. 



This species may be known by its truncate anterior end. The form of the 

 body is very similar to L. extenuation, but it is somewhat more convex and a 

 little wider; the wing margin is less concave and the extremity less extended. 

 The right valves of these two species are conspicuously different. 



Formation and locality. In the central portion of the Chemung group. Mans- 

 field, Tioga county, Pa. 



