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l8o NEW YORK STATE MUSEUM 



Genus leptaena Dalman 



[Ety.: Xevros, thin] 



(1828. Kongl. Svenska Vet, Akad. Handl. p. 93,94) 



Shells concavo-convex; surface covered by conspicuous concen- 

 tric corrugations or wrinkles over the flatter portions of the valves. 

 Where these cease, the surface is more or less abruptly deflected, 

 forming a conspicuous anterior slope. Whole exterior covered with 

 fine, radiating, tubular striae, which in well preserved specimens are 

 crenulated by finer concentric striae. Hinge line straight; cardinal 

 area narrow. A convex deltidium present, perforated at the apex 



by a foramen, which often en- 



- — 7 ; croaches on the apex of the valve. 



, ./ A trilobed cardinal process and 



well defined muscular impressions 

 are present. 



Leptaena rhomboidalis (Wah- 

 lenberg) (Fig. 84). Leptaena 

 depressa Hall (1852. Pal. 

 N. Y. 2 :257, pi. 53, fig. 6a-l) 

 Distinguishing characters. Cor- 

 Fi g . 84 Leptaena rhomboidalis rugated part gently convex to 



slightly concave; abrupt anterior deflection; strong concentric cor- 

 rugations and fine striae. Narrow hinge area. 



Found in the upper Clinton limestone, the Clinton lenses, and 

 the lower Rochester shale up to and in the Bryozoa bed. Rarely 

 above this. Also at Lockport and elsewhere (Hall). 



Genus stropheodonta Hall 

 [Ety.: arpocprj, bend; odous, tooth] 



(1852. Pal. N. Y. 2:63. Hall & Clarke, 1892; Pal, N. Y. v. 8, 

 pt 1, p. 284) 



Shell normally concavo-convex; hinge line usually equal to or 

 greater, than the greatest width of the shell. Area of the pedicle 

 valve higher than that of the brachial valve, both furnished with 

 projecting denticulations, which interlock and form articulations. 

 Muscular areas well marked and variously bounded. A strongly 

 marked bifid cardinal process occurs in the brachial valve. 



