LAMELLIBRANCHL IT. I. 177 



Test thin, marked by closely arranged concentric striae, which at irregular 

 intervals are crowded into fascicles, producing a gently undulated Burface. 

 On the wing the striae are closely arranged, and just below the hinge-line 

 are turned backward along the spiniform extension of the wing. 



Interior unknown. Ligamental area narrow, having several fine.grooves 

 parallel to the hinge. 



One specimen has a length from Weak to base of 24 mm., heighl I 5 mm., 

 hinge-line about 2<> mm. The corresponding ri- lit 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 le>> than 10 mm. in length and 

 height. 



This species, occurring abundantly in the Hamilton group, closely resembles 

 Avicula -spinigera of Conrad, from the Chemung group. In the comparison of 

 large numbers of specimens, the differences are so slight that specific distinc- 

 tion is not always apparent. The Chemung forms are usually more oblique, 

 but this is not a constant character: 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 righl 

 valve of the Hamilton form. 



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

 Chenango county; Middleville, Schoharie county, and Leonardsville, Madison 

 county, N. Y. 



LEPTODESMA SPI.XIGERUM. 



PJLATE XXI, FIGS. 1" 1 : : nn PLATE I.XXXIX. FIG. 1 



Avicula spinigera, Cohrad. Join-. Acad. Nat. Sri.. Phila., vol. 8, p. ■2:'~. pi. 12, fig. :!. IS42 



Pteronites spinigerus (Comrad), S. A. Miller. Cat. Amer. Pal Foss., p. 202. 1~-T7. 



Leptodesma spinigervm (Conrad), Hall. Pal. N. Y.. veil. v. pt. 1. Plates and Explanations: PI. 21, figs. 



10-13. Jan.. l--s::. 



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

 45 

 23 



an'de of about 45°: length one-third greater than the height : ante-byssal 



