NIAGARA FALLS AND VICINITY 



195 



Rhynchonella (?) bidentata (Hisinger) (Fig. 11 1). A try pa 

 bidentat-a Hall (1852. Pal. N. Y. 2:276, 



Pl- 57) 



Distinguishing characters. Triangular form; 



acute, extended beak of pedicle valve; stronger 



convexity of brachial valve; less convex and more 



triangularly acute than preceding; very slight 



- , . . 1 11 . -1 ,. Fig. in Rhynchcnel- 



irontal emarg-mation ; shallow smus with one ph-ia n) bidentata with 



^ surface enlarged 



cation, and corresponding fold with two. 



Found in the Rochester shale at Lockport and elsewhere (Hall). 

 Probably also at Niagara. 



Genus atrypa Dalman 

 [Ety.: a, without; rpo-a, foramen (erroneous)] 

 (1828. Kongl. Vetenskaps Akad. Handlingar, p. 127; 1894. Pal. 

 N. Y. V. 8, pt 2, p. 163) 



Shell varying in outline from nearly circular to longitudinally 

 suboval; valves very unequal, brachial valve being strongly convex 

 or gibbous, while the pedicle valve is gently convex or almost flat 

 or sometimes slightly concave from the strongly marked sinus; 

 beak of the pedicle valve small and incurved over that of the 

 brachial. Large widely separated and doubly grooved teeth are 

 present, unsupported by lamellae. Strong muscular impressions. 

 Spirals of the brachidium with their bases parallel to the inner sur- 

 face of the pedicle valve, and the apexes directed toward the deepest 

 point of the opposite valve. Surface radially plicate. 



Atrypa reticularis (Linnaeus) (Fig. 112) (1852. Pal. N. Y. 

 2:272, pi. 23, p. 270, ph. 55) 



Fig. 1 12 Atrypa reticularis 



Distinguishing characters. Convex brachial and flat pedicle 

 valves; small deeply incurved beaks; radiating and concentric striae 

 forming reticulated surface. 



