NIAGARA FALLS AND VICINITY 183 



subauriculate ; cardinal areas narrow, sometimes obscurely crenu- 

 lated on the margins. Delthyrium partially closed by convex plate, 

 but mostly occupied by cardinal process of opposite valve. Car- 

 dinal process appears trilobate. Muscular areas moderately well 

 defined. 



Plectambonites sericea (Sowerby) (Fig. 89). Leptaena 

 s e r i c e a Hall (1852. Pal. N. Y. 

 2:59, pi. 21) 1 > -\ ' '■ .■ ' 



Distinguishing characters. Outline ■, ,\ v \\ 



semicircular to semioval ; hinge line ex- 

 tended, ending in acute points; Striae Fig. 89 Plectambonites set icea 



strong, elevated, alternating with finer ones. 



Found in the lower Clinton limestone at Niagara. Rare. 



Plectambonites transversalis (Wahlenberg) (Fig. 90). Lep- 

 taena trans versalis Hall (1852. Pal. N. Y. 2 \2.^o, pi. 



53; fi g- 5a-b) 



Distinguishing characters. Out- 

 line semicircular; strongly convex 

 pedicle, and extremely concave bra- 

 chial valve, conforming to each other; 

 strongly incurved beak of pedicle 

 valve, which causes an inflection of 

 the hinge line; hinge line produced; 



Fig. 90 Plectambonites transversalis - , . , , . . . 



fine distant elevated striae with ex- 

 tremely fine striae between; strongly punctate character of ex- 

 foliated portions. 



Found in the Clinton lenses, and abundantly in the lower Roches- 

 ter shale at Niagara. Also at Lockport (Hall). 



Genus orthothetes Fischer de Waldheim 



[Ety. : 6pd6$ ; straight] 



(1830. Soc. imp. natural, d. Moscow Bui. 1:375. Hall & Clarke, 

 1892; Pal. N. Y. v. 8, pt 1, p. 253) 



Shell varying from plano-convex to biconvex, sometimes becom- 

 ing concavo-convex with age. Pedicle valve most convex about 

 the beak, which often tends toward irregular growth; cardinal area 

 well developed, with a thick, more or less convex deltidium. Teeth 

 not supported by dental plates. Brachial valve most convex near 



