Lamellibrastchiata. 349 



Test thin, marked by closely arranged concentric stria3, which at 

 irregular intervals are crowded into fascicles, producing a gently 

 undulated surface. On the wing the stride are closely arranged, and 

 just below the hinge-line are turned backward along the spiniform 

 extension of the wing. Ligamental area narrow, having several fine 

 grooves parallel to the hinge. 



One specimen has a length from beak to base of 24 mm., height 

 15 mm., hinge-line about 20 mm. The corresponding right valve 

 has a length of 20 mm., height 15 mm., hinge-line 26 mm. The 

 largest specimen observed has a length of 32 mm., height 21 mm., 

 hinge-line, to the origin of the spine, 20 mm. Small specimens are 

 often less than 10 mm. in length and height. 



This species, occurring abundantly in the Hamilton group, closely 

 resembles AvicuTa spinigera of Conrad, from the Chemung group. 

 In the comparison of large numbers of specimens, the differences are 

 so slight that specific distinction is not always apparent. The Che- 

 mung forms are usually more oblique, but this is not a constant charac- 

 ter ; the wing is smaller and less extended along the posterior slope ; the 

 right valve is narrower and more oblique, and the sinus in the margin 

 of the wing is narrower and deeper than in the right valve of the 

 Hamilton form. 



Formation and localities. In the shales of the Hamilton group at 

 Norwich, Chenango county, Middle ville, Schoharie county, and 

 Leonardsville, Madison county, N. Y. 



Leptodesma spinigerum. 



Avicula spinigera, Conrad. Jour. Acad. Nat. Sci., Phila., vol. 8, p. 237, pi. 12, 



fig. 3. 1842. 

 Pteronites spinigerus (Conrad), S. A. Miller. Cat. Amer, Pal. Foss., p. 202. 



1877. 

 Leptodesma spinigerum (Conrad), Hall. Pal. N. Y., vol. v, pt. 1. Plates 



and Explanations : PI. 21, figs. 10-13. Jan., 1883. 



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

 hinge at an angle of about 45° ; length one-third greater than the 

 height ; ante-byssal margin truncate at the extremity, curving be- 

 low into the sinus; basal margin rounded, somewhat abruptly 

 recurved posteriorly. Left valve regularly convex below, gibbous 

 above the middle. Right valve depressed-convex below and abruptly 

 gibbous above. Hinge-line straight (and including the spine), 

 equal to or greater than the length of the shell. Beaks sub-ante- 

 rior, prominent. Umbonal region abruptly gibbous, subtending an 



