LINGUL^ OP THE HAMILTON GROUP. 11 



This shell resembles L. punctata in form, but is proportionally narrower : it is 

 truncate in frout in the same manner; the umbo is a little narrower, and it has not 

 the flattened space below the middle so distinctly indicated. The surface marking, 

 however, is always a more distinctive character. The largest specimen examined, 

 little exceeds half an inch in length. 



Geological formation and locality. In the shales of the Hamilton group, asso- 

 ciated with L. punctata : near Monteith's Point, on Canandaigua lake. 



Lingala densa ( n. s.). 



PLATE II. 



Shell subelliptical ; sides almost exactly parallel for more than half the 

 length of the shell, abruptly curved at the basal angles, and the base 

 nearly straight or slightly curving ; cardinal margins gently curving. 

 Middle and upper part of the shell prominent and rounded ; one valve 

 (ventral?) showing a slight longitudinal depression when exfoliated, 

 with fine wrinkled muscular impressions. Valves somewhat abruptly 

 flattened and compressed below the middle. 



Shell comparatively thick, compact, and very closely and finely striated 

 concentrically, with undulating striae, the exfoliated shells showing 

 radiating striae. 



This species has nearly the same form as L. punctata^ but is more robust and more 

 abruptly elevated along the middle of the upper half of the valve : the flattening 

 is only near the lower extremity, and does not extend upwards in a triangular form 

 as in that species. The absence of puncta, and the close scarcely elevated fine striae, 

 are distinguishing features. 



Geological formation and locality. In the .upper part of the Hamilton group, 

 near Summit, Schoharie county; occurring in large numbers in some semi-arenaceous 

 layers on the small stream flowing in a gorge known as " Bear Gulf." 



