CENTRONELLA OF THE HAMILTON GROUP. 403 



This species has the general form and aspect of C. glans-fagea ; differing, 

 however, in a very characteristic manner. It has a similar compact and finely 

 punctate texture, and the propriety of referring it to the same genus can scarcely 

 be doubted. 



Geological formation and localities. This species occurs in the Hamilton group : 

 on Coshong creek near Bellona in Yates county ; at York in Livingston county ; 

 at Pavilion in Genesee county ; and at Hamburgh on the shore of Lake Erie, iu 

 Erie county. 



Ceiitrunella glaucia (n. s.). 



PLATE LXI. 



Shell ovate or ovoid, gibbous, with the front rounded or subtruncate 

 Ventral valve gibbous, highly arcuate longitudinally, more elevated in 



the middle and sometimes subcarinate, curving abruptly to the sides : 



beak strongly arched. 

 Dorsal valve convex on each side and abruptly bent downwards at the 



margins, slightly depressed in the middle of the upper part, with a 



more distinct sinus towards the front. 

 Surface marked by fine concentric striae, which at intervals are crowded 



into subimbricating folds or ridges. Shell compact; structure punctate. 



The cast of the ventral valve shows a deep arching rostral cavity, 

 and strong muscular imprints below, with deep curving dental lamellce. 



The specimens are nearly three-fourths of an inch in length, with a 

 width of more than half an inch. 



Geological formation and locality. This species occurs in some compact beds 

 of the Hamilton group in Schoharie county; the particular locality unknown. 



