456 



NORTH AMERICAN INDEX FOSSILS. 



teeth and a deep ligament area whose sides are marked with paral- 

 lel grooves. Devonic. 

 255. P. knappianum Hall and Whitfield. (Fig. 606, a-c.) 



Devonic. 



Valves convex below and gibbous above. Concentric striae 

 interrupted by occasional growth varices. Externally resembles 

 some forms of Modiomorpha but is distinguished by the deeply 

 grooved ligament area characteristic of the genus. Type of the 

 genus. 



Hamilton group : New York, Ohio, Falls of Ohio. 



LXXVII. Modiella Hall. 

 Equivalve, very inequilateral, with short anterior end and very 

 broad and curved posterior end. Hinge line arcuate. Surface 

 covered with fine concentric striae. Hinge area longitudinally 

 grooved. Pallial line simple. Very similar to recent species of 

 Modiola but lacking the pronounced epidermis. Devonic. 



FlG. 606. a-c, Ptychodesma knappianum, right valve, ligament area enlarged, and 

 cardinal view ; d, Modiella pygma>a, X 2> 



256. M. pygmaea (Conrad). (Fig. 606, d.) Devonic. 



Basal margin indented anterior to middle owing to the depres- 

 sion just anterior to the beak and extends to the base. Type of 

 the genus. 



Hamilton : New York, Pennsylvania. 



LXXVIII. Aucella Keyserling. 

 Obliquely ovate, with anterior beaks. Left valve larger and 

 arcuate, with a small ear set off from the body of the shell by a 



