DIPI.ODON 1225 



the other, and two laterals in the left valve; nacre bluish to 

 white, dull, often blotched ; beak cavities shallow ; dorsal scars 

 numerous, forming a row in the beak cavity parallel with the 

 hing-e line. 



Animal with the marsupium occupying nearly the whole 

 length of the inner ]:iranhi?e, a few ovules sometimes being 

 foimd in the outer gills ; branchicX rather large, angular at base, 

 inner much the larger, united their whole length to the ab- 

 dominal sac ; palpi scarcely or somewhat projecting posteriorly ; 

 mantle very thin, thickened on the edges; branchial opening 

 papillose, separated from the smooth anal opening by a strong 

 bridge : superanal opening not closed below. 



Type, DJhlodoii clJipticnin Spix. 



Kf;y to groltps of Diplodon. 



On account of the similarity of characters of a large propor- 

 tion of the forms of this genus and the fact that the beak sculp- 

 ture is eroded away in nine-tenths of the specimens, it does 

 not seem possible to me to construct a key that will lead to 

 the species. For the saiiie reasons I fear that the following 

 key to the groups will not prove very satisfactory. 

 Shell generally short. 



Having radial beak sculpture. 

 Obovate to suborbicular. 



Moderately solid. Sect. Cyclomya. 



Rather thin. Sect. Bulloideus. 



Subquadrate, inflated. Group of D. quadrans. 



Without radial beak sculpture. Subgenus Lccznrostris. 



Shell more or less elongated. 

 Beak sculpture of broken lines. 

 vShell elliptical or subtrapezoid. 



Kather solid, bars of beak sculpture strong, slight- 

 ly coalescing below. Group oi' D. lacteolus. 

 Thin or scarcely solid, bars of beak sculpture nar- 

 row, not coalescing. Group of D. grmiosns. 

 Posterior ridge high ; shell pointed behind ; beak 

 sculpture of strong narrow ribs. 



Group of D. hiirrous!,hianits^ 



