268 



PROCEEDINGS OF THE MALACOLOGICAL SOCIETr. 



old shells a rough irregular ridge. This inner ridge is still more 

 prominent in T. nitidxda (Sow., non Lam.). 



T. intermedia (Sow.) furnishes a good example of the next stage in 

 the development of accessory teeth, for in this species the nymph is 

 much thickened and divided into two parts by a deep central groove, 

 so that tliere are two distinct dentiform ridges. Moreover, the three 



Fig. 1. Tivela damaoides, Gray. 

 2. ,, mactroides (Born). 

 ,, 3. ,, argent ina (Sow.). 

 ,, 4. ,, gracilior (Sow.). 



,, 5. Grateloiqna irregularis, Bast. 



normal teeth are crowded into the anterior lialf of the cardinal plate; 

 the anterior cardinal, though visible in young specimens, becomes 

 obsolete with age, the median is narrowly triangular and directed 

 forward, while the posterior is very narrow, short, straight, and 

 almost vertically central. 



In the left valve the posterior cardinal is also central, rather thick, 

 and bifid when young; the nyniphal area beyond it is thickened, 



