i84 



NEW YORK STATE MUSEUM 



the middle, with a narrow hinge area; cardinal process quadrilobate 

 as seen from above. Surface covered by slender radiating striae^ 

 which are crenulated by concentric lines. 



Orthothetes subplanus (Conrad) (Fig. 91). Leptaena sub- 

 plana Hall (1852. Fa/. A^. F. 2:259, pi. 53, fig. 8-10) 



Distinguishing characters. Pedicle valve at first convex, later be- 

 coming concave; valves nearly equal in length and width; extended 

 hinge line, sometimes projecting into points; sharp angular or sub- 

 angular to rounded striae, sometimes bifurcating before reaching 





Fig. 91 Orthothetes subplanus 



margin, separated by wider interspaces ; the usual method of increase 

 is by intercalation of fine striae, which soon grow to strength of 

 the chief ones; fine concentric and occasionally coarser lines of 

 growth. 



Found at Niagara in the upper Clinton beds and the Clinton 

 lenses; also abundantly in certain thin calcareous layers of the lower 



Rochester shale, less common in 

 the middle and upper shale. Also 

 at Lockport and elsewhere (Hall). 

 Orthothetes hydraulicus (Whit- 

 field) (Fig. 92) (Grabau. GeoL 

 soc. Am, Bui. II :365, pi. 22) 

 Distinguishing char actors. Small 

 size; obtuse cardinal margins with hinge line shorter than greatest 

 width of shell; uniformly rounded front; strong rounded, sharply de- 



Fig. 92 Orthothetes hydraulicus 



