FOSSILS OF THE NIAGARA GROUP. 383 



LAMELLIBRANCHIATA. 



The following species, including several heretofore described, have 

 been recognized in the Niagara limestone of Wisconsin and adjacent parts 

 of Illinois. 



GENUS AMBONYCHIA, Hall. 

 Ambonychia aph^a, n. s. 



PLATE XIV, FIG. 3. 



Shell somewhat obliquely ovate ; anterior side broadly rounded ; beak 

 in the cast acute ; hinge-line straight, and about half the length of 

 the shell. The posterior basal extremity is somewhat abruptly 

 rounded. Muscular impressions large, situated below the centre of 

 the length of the shell. The greatest width of the shell is equal to 

 about two-thirds the length. 



This species is proportionally broader and shorter, with the beaks less 

 extended than A. acutirostra from the same horizon. 



One specimen measures one inch and seven eighths in length and one 

 inch and a quarter in width. Other specimens are smaller than the one 

 measured. 



Formation and Locality. — In limestone of the age of the Niagara group, 

 at Wauwatosa, Wisconsin, and Bridgeport, Illinois. 



Ambonychia acutirostra, Hall. 



PLATE XIV, FIG. 2. 

 Amhonychia mytiloides, Hall. Rep. Progress Geol. Survey of Wisconsin for 1859. 1860. 



This species is proportionally longer than the preceding, less expanded 

 on the anterior margin, and has the beaks elongate and attenuate. In 

 many respects it resembles the Myalina mytilifonnis of the grey sandstone 

 of the Clinton group. 



These species are probably not true Ambonychia, but in the condition 

 in which they occur, we have not sufficient characters remaining, to 

 authorize their separation. 



Formation and Locality. — In limestone of the age of the Niagara group, 

 near Milwaukee, and at Wauwatosa and Racine, Wisconsin. 



