Lamelltbranchiata. 345 



The two specimens observed are crushed, and the characters of the 

 left valve are very much obscured, while the right valve preserves more 

 nearly its true proportions. It differs from the other species of this 

 group in the concave form of the right valve. In surface markings it 

 is very similar to L. Sayi, but the concave right valve is a very dis- 

 tinguishing feature. The shell is large and thin, and has suffered 

 compression and distortion in the process of imbedding in the soft 

 shales. 



Formation and locality. In the soft shales of the Hamilton group, 

 Oanandaigua lake, Ontario county, N. Y. 



. Leiopteria Chemungen-sis. 



Avicula Chemung ends, Vanuxem. Geolog. Surv. N. Y. : Rep. Third Dist. , p. 



182, fig. 49, No. 3. 1842. 

 Leiopteria nitida, Hall, Pal. N. Y., vol. v, pt. 1. Plates and Explanations : 



PL 22, figs. 17, 18. Jan., 1883. 



Shell of medium size or larger, rhomboid-ovate; body elongate-ovate, 

 oblique at an angle of about 60° with the hinge-line ; length equal 

 to more than four-fifths of the height; anterior margin nearly ver- 

 tical for about one-half the height of the shell and broadly rounded 

 on the base; post-basal margin abruptly rounded. Left valve con- 

 vex, gibbous in the umbonal region. Right valve smaller, less gib- 

 bous. Hinge-line straight, a little longer than the height of the 

 shell. Beaks sub-anterior, inclined forward, acute, prominent, arch- 

 ing over the hinge. Umbonal region narrow, prominent, subtend- 

 ing an acute angle. Ear of left valve small, bending slightly down- 

 ward, defined by abroad distinct sulcus ; extremity rounded. Ear 

 of right valve flat. Byssal sinus shallow and broad, deeper in the 

 right valve. Wing broad-triangular, joining the body below the 

 middle of the length, limited by the abrupt bending of the striae ; 

 margin deeply concave in the middle; extremity produced and 

 acute. 



Surface, in a partial cast, marked by fine concentric striae, which 

 are crowded and lamellose on the ear, distinctly marked on the wings, 

 and appearing on the "body as distinct undulations ; these in the 

 perfect shell may have been sharp lamelliform striae. 



A specimen of the left valve has a length of 30 mm., height 35 

 mm., hinge-line 47 mm.; the body, from beak to post-basal extremity, 

 measures 43 mm. 



The specimen described is the original of Avicula Chemung ensis, 

 Vanuxem. It resembles L. Dekayi of the Hamilton group, but differs 

 [Sen. Doc. No. 38.1 44 



