196 



NEW YORK STATE MUSEUM 



Found in all the beds from the upper Clinton limestones to the 



Bryozoa beds. Most abundant in the Clinton. 



In the light colored crystalline upper Clinton limestone this 

 species is very abundant, but also very variable. Strong robust and 

 very rotund specimens occur, with brachial valve excessively bulg- 

 ing, and with narrow simple rounded striae, increasing by implan- 

 tation, and cancelated by concentric striae of moderate strength, and 

 stronger undulations on the mature portions. Other specimens, less 

 rotund and with bifurcating striae occur. When bifurcation of 

 striae occurs, this is usually found on the pedicle valve, those of the 

 brachial valve increasing only by intercalation. The pedicle valve 

 usually has a sinus near the front, and the striae and concentric lines 

 increase in strength, approaching the characters of the next species. 

 In the Clinton lenses this character becomes still more pronounced, 

 the shells at the same time decreasing in size and rotundity. In the 

 Rochester shales the species is generally much less abundant, A. 

 nodostriata being the prominent form. A. reticularis 

 is represented by small and generally flattened specimens, in which 

 the radiating striae are usually fine, and the concentric striae 



lamellose, specially in the adult por- 

 tions. Increase of striae occurs by 

 both intercalation and bifurcation, 

 the former on the brachial, the lat- 

 ter on the pedicle valve. In some 

 specimens the bifurcation occurs close 

 to the beak. 



Atrypa nodostriata Hall (Fig. 113) 

 (1852. Pal. N. Y. 2:272, pi. 56) 



Distinguishing characters. Sub- 

 equal valves, nearly equally convex in 

 young, pedicle valve more convex 

 with age; small slightly elevated beak 

 enlarged f pedicle valve ; mesial sinus in adult 



shells, broad and undefined; strong rounded bifurcating plications; 

 lamellose growth lines which give nodulose appearance to surface. 

 Found in the Clinton lenses, and the lower and middle Rochester 

 shale at Niagara. Specially abundant in the Bryozoa beds. Also 

 found at Lockport and elsewhere (Hall). 

 Atrypa rugosa Hall (Fig. 114) (1852. Pal. N. Y. 2:271, pi. 56) 

 Distinguishing characters. Generally smaller than preceding; 

 equally convex valves in adult, unequal in young, the brachial valve 



Fig. 113 Atrypa nodostriata, with striae 



