THE NAUTILUS. 45 



a 2 Lateral hinge-teeth obsolete or absent. Beak sculpture 

 more or less heavy, with or without a sinus. Inner 

 lamina of inner gills with the tendency to become more 

 or less united to the abdominal sac. 



b 1 Posterior ridge of shell indistinct or blunt, posterior 

 slope not distinctly truncated, without corrugations, 

 or only with faint traces of them. Cardinal teeth 

 strongly developed, triangular or squarish, stumpy. 

 Epidermis with unbroken rays. 



c 1 Beak sculpture moderately heavy, subconcentric. 

 Shell subrhomboid, posterior ridge moderately de- 

 veloped, blunt, posterior angle of shell little ele- 

 vated above base line. Female shell distinguish- 

 able by a slight swelling of the posterior ridge, 

 accompanied by a radial depression of the poster- 

 ior slope, causing a shallow emargination of the 

 posterior margin. 



Subgenus: Pressodonta Simpson. 

 (Type: A. calceolus (Lea)). 



c 2 Beak sculpture very heavy, bars straight or with a 

 slight indication of a sinus in front of the posterior 

 angle. 



d 1 Shape of shell regularly ovate, with posterior and 

 moderately elevated above the basal line, and 

 upper and lower margins converging rather 

 uniformly toward it, without a truncation. Pos- 

 terior ridge indistinct. Male and female shells 

 undistinguishable. 



Subgeuus: Alasmidonta Simpson. 

 (Type: A. undulata (Say)). 



d' 2 Shape of shell irregularly subovate, with poster- 

 ior end greatly elevated above base line, and an 

 oblique truncation below this end. Posterior 

 ridge present. Female shell distinguishable by 

 the stronger development of the posterior ridge 

 and the greater obliquity of the postero-basal 

 truncation, which is slightly emarginate. 



Subgenus: Pegias Simpson (as genus). 

 (Type: A. fabula (Lea)). 



