52 



NORTH AMERICAN INDEX FOSSILS. 



from 2 to 3 mm. in adult; living chamber broadly constricted, 

 anterior to the middle. 



Hamilton of New York and Maryland. 



30. 0. exile Hall. (Fig. 1252, a.) Devonic. 

 Differs from the preceding in its excentric siphuncle, and rare, 



faint constriction of living chamber. 

 Hamilton of New York and Maryland. 



31. 0. eriense Hall. (Fig. 1253.) Devonic. 



Fig. 1 25 1. Orthoceras con- 

 strictuvi, X K- (After Grabau. ) 



Fig. 1252. a^ Orthoceras exile, y(^ 2/^-, b, 

 O. ( Geisonoeeras^ subulatum, X z^- (After 

 Grabau. ) 



Large, straight, and robust, regularly and gradually enlarging 

 to the slightly constricted aperture; section circular; apical angle 

 8°; septa moderately concave (arc of ii6°) ; siphuncle central; 

 surface with concentric and longitudinal striae. 



Hamilton of New York. 



32. 0. (Geisonoceras) subulatum Hall. (Fig. 1252, &.) Devonic. 

 Rather rapidly expanding, with circular section; siphuncle sub- 



