LAMELLIBRANCHIATA. 177 



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

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

 On the wing the stri;>? are closely arranged, and just below the hinge-line 

 afe turned backward along the spiniforni extension of the wing. 



Interior unknown. 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 tlie 

 spine, 20 mm. Small specimens are often less than 10 nnii. in length and 

 height. 



This species, occurring abundantly in the Hamilton gioup, closel}' resembles 

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

 large numbers of specimens, the diflferences 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 oblicjue, 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 ; Middleville, Schoharie county, and Leonardsville, Madison 

 county, N. Y. 



Leptodesma spimgerum. 



PLATE XXI, FIGS. 10-13; AXD I'LATE LXX,XIX, FIG. 1. 



Avicula spinigera, Conrad. .Jour. Acad. Nat. Si-i., Phila., vol. S, ]i. 2:-!7, jil. 12, tig. 8. 1S42. 

 Ptenmite-s 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, tig-s. 



10-13. Jan., 1883. 



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

 angle of about 45° ; length one-third greater than the height ; ante-byssal 



