312 NORTH AMERICAN INDEX FOSSILS, 



y 



e f 



Fig. 392, a-c. Atrypa reticularis; a, interior of pedicle valve ; b, cardinal portion 

 of brachial valve ; c y spirals showing after removal of brachial valve ; d, Atrypa spinosa ; 

 e, Atrypa hystrix ; /, A. hystrix var. occidentalis. a, adductor scars ; b, dental 

 sockets ; c, crural bases ; hp, hinge plate ; /, pedicle scar ; ;-, diductor scars ; sr, 

 crenulated ridge in sockets. (After Hall and Clarke. ) 



C. Cyrtina Davidson. 

 Small, spiriferoid, with very unequal valves. Differs from Spirifer 

 in the semipyramidal form of the pedicle valve, the high, flat car- 

 dinal area and the narrow delthyrium closed by a convex pseudo- 

 deltidium which is usually perforated by a circular foramen. Shell 

 structure punctate. The dental plates converge from the inner 

 margins of the delthyrium and unite with the median septum, 

 which reaches to the front, thus dividing the interior of the valve 

 into two parts. Siluric-Mississippic. 



A. Cardinal area of pedicle valve strongly arched *, 



* Shell large, more than ]/ z inch wide 321. C. alpmaensis. 



* Shell small, less than l / 2 inch wide 320. C. umbonata. 



