NIAGARA FALLS AND VICINITY 195 



Rhynchonella (?) bidentata (Hisinger) (Fig. in). Atrypa 

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



Pi- 57) 



Distinguishing characters. Triangular form; 

 acute, extended beak of pedicle valve; stronger 

 convexity o»f brachial valve; less convex and more 

 triangularly acute than preceding; very slight 



. . . ... . . t ,. Fig. Ill Rhynchcnel- 



frontal emargmation; shallow sinus with one pli-ia (?) 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, pi. 55) 



Fig. 112 Atrypa reticularis 



Distinguishing characters. Convex brachial and flat pedicle 

 valves; small deeply incurved beaks; radiating and concentric striae 

 forming reticulated surface. 



