PELECYPODA—PRIONODESMACEA. 



43i 



185. A. alata (Meek). [Ambonychia alata.) Ordovicic. 



Outline triangular with hinge line nearly at right angles to the 

 anterior margin. 24-28 strong, radiating ribs present. 



Upper part of the Cincinnati Group of Ohio, Indiana. 



LVI. Byssonychia Ulrich. 

 Resembles Ambonychia but differs in its more erect form and in 

 the presence of a byssal opening in the upper half of the anterior 



Fig. 562. a, b, Byssonychia intermedia; 

 c, B. radiata (/ = pallial line). (Minn. 

 Surv. ) 



Fig. 563. Byssonychia ? byrnesi. 

 Ohio, VII.) 



Pal. 



side and in the presence of two or three slender lateral teeth situ- 

 ated posteriorly, as well as several cardinal teeth. Radiating lines 

 usually stronger than in Ambonychia. Ordovicic. 



186. B. intermedia (Meek and Worthen). 

 (Fig. 562, a, b.) Ordovicic. 

 Small. Beaks strongly incurved, with a 



slight forward direction. 



Galena : Illinois, Wisconsin, Minnesota, 

 Lake Winnipeg,. Canada. 



187. B.? byrnesi Ulrich. (Fig 563.) 



Ordovicic. 



The hinge is peculiar, having no lateral teeth and only one 

 cardinal. It differs externally from B. acutirostris (Fig. 564) in 

 the byssal opening not being impressed; hinge line slightly longer; 

 radii 20 to 22 and slightly curved, instead of 36 to 40 and straight 

 as in B. acutirostris. 



Utica : Kentucky, Tennessee. 



Fig. 564. Byssony- 

 chia acutirostris. (After 

 Ulrich, Pal. Ohio, VII. ) 



