Lamellibranchiata. 355 



The largest specimen observed has a length of 26 mm., height 

 13 mm., hinge-line 20 mm. A similar right valve has a length of 

 23 mm., height 10 mm., hinge-line 16 mm. The specimens are 

 usually smaller than those figured. 



This species resembles in form of body and in the concentric striae, 

 L. longispinum ; but the anterior extremity (especially in the right 

 valve) is narrower, the wing is extended farther down the body of the 

 valve, and the spine is less produced. This shell in its usual condition 

 of preservation (that is, having the spiniform process of the wing 

 broken off, or the wing partially covered) presents the characters de- 

 scribed by Mr. Conrad, although not illustrated in his figure. A 

 specimen of this species is labeled in the handwriting of Mr. Conrad, 

 Avicula protexta, and therefore the name is retained. 



Formation and localities. In the Chemung group at Chemung Nar- 

 rows, N. Y., and Tioga county, Pa. 



Leptodesma Becki. 



Leptodesma Becki, Hall. Pal. N. Y., vol. v, pt. 1. Plates and Explanations ; 

 PI. 22, figs. 3-5 (and figs. 6 and 7 in error). Jan., 1883. 



Shell of medium size, sub-rhomboidal ; body oblique, narrowly ovate 

 in the left valve ; length more than one-third greater than the 

 height; anterior margin oblique, extending in nearly a direct line 

 into the broadly rounded basal margin ; post-basal margin produced, 

 not abruptly recurved into the wing. Left valve gibbous except in 

 the lower part. Eight valve less convex. Hinge-line straight, less 

 than the length of the shell, and greater than the height. Beaks 

 at about the anterior third of the hinge-line, obtuse, directed for- 

 ward and arching over the hinge. Umbonal region gibbous, scarcely 

 defined anteriorly, but well-marked posteriorly by the abrupt slope 

 of the side. Umbonal angle acute. Anterior end truncate, acute 

 at the extremity. Wing of medium size, not distinctly limited, 

 joining the body nearly at the posterior extremity ; margin concave; 

 extremity produced into a short spine. The right valve is propor- 

 tionally higher, the body less distinctly defined, and the wing 

 larger. 



Test thin, marked by concentric striae, which are crowded and 

 somewhat lamellose in front, and closely and evenly arranged on the 

 wing. The hinge-line has a single narrow groove. 



A left valve has a length of 28 mm., height 17 mm., hinge-line 



