250 



obliquely from beak to base, and the surface is marked by 

 rugose, or undulating, concentric lines. The hinge bears a 

 strong tooth in the lefl valve, which fits into a cavity in the 

 right valve. The ligament is external and is attached to the 

 thickened margin of the shell, which is often longitudinally 

 grooved for its reception. 



Modiomorpha concentrica. (Conrad.) (Fig.165.) (Pal. 

 \. V.. Vol. V.. Pt. I.. p. 27o. Pis. XXXIV.. XXXV.. XXXVI.) 



Distinguishing Clianatrrs—Qynte outline: straight or 

 slightly concave basal margin; arcuate to straight cardinal 



Fig. 165. Modiomorpha concentrica. Right side of a large individual, showing the 

 arcuate form characteristic of old individuals (from Hall . 



margin; rounded, projecting, well-defined, anterior end; 

 strong, regular concentric lines. 



Found in the Encrinal limestone, at Section 5 (common); 

 in the shale, at twenty-five feet below the Encrinal and be- 

 low the Trilobite beds, and in the Pleurodictyum beds, on the 

 Lake Shore l rare). 



Modiomorpha rub-alata. (Conrad.) (Fig. 166.) (Pal. 

 V V.. Vol. V.. Pt. I., p. 283, Pis. XXXV.. XXXIX.) 



Distinguishing Characters. — Sub-quadrangular to sub- 

 ovate outline; sub-alate posterio-dorsal portion: well- 

 marked umbonal ridge, passing from beak to base; large, 

 but ill-defined, anterior end; sub-truncate posterior end. 



Found in the shale below the Demissa bed and below the 

 Athyris bed (rare); also in the Modiomorpha sub-alata bed, 

 in Idlewood Ravine, and on the Lake Shore (in great 



