259 



Distinguishing Characters.— Deeply sinuate or eraarginate 

 posterior margin; strong umbonal ridge, with depression 

 above it; produced post-cardinal extremity; strong, ele- 

 vated, distant, lamellose, concentric ridges, with finer stria? 

 between. 



Found in the Lower shales, down to twenty-five feet below 

 the Encrinal limestone, at Sections 5 to 7 (common); also 

 in the shale below the Nautilus bed, in Avery's Creek. 



Genus MACRODON. Lycett. 



[Ety. : Macros, long ; odous, tooth.] 

 (1845: Murch. Geol. Chelt.) 



Shells with equal inequilateral valves, which are trans- 

 versely sub-elliptical, or sub-ovate, in outline. The anterior 

 end is angular at the cardinal line and rounded below. The 

 posterior end is rounded or obliquely sub-truncate. Beaks 

 anterior to the middle; cardinal line long and straight. 

 Ligament external. Cardinal teeth several; lateral teeth 

 two to four, situated near the extremity of the cardinal line. 

 Surface marked by concentric stria?, which are often lamellose, 

 and in some species by tine radiating lines. 



Macrodon hamilton:le. Hall. (Fig. 178.) (Pal. N. Y., 

 Vol. V., Pt, I., p. 349, PL LI.) 



Distinguishing Characters. — Obtusely sub-angular ends 

 of hinge line ; rounded post-inferior end ; strong, distant 



Fig. 178. Macrodon hamiltonice. A right valve ; a specimen retaining both valves, the 

 shell exfoliated (from Hall;. 



lamellose concentric lines; fine interrupted radii, strongest 

 on the posterior part of the shell. 



