No. 470] 



OLD AGE IN BR AC H 10 POD A 



99 



specimen of Rhijnchotrema capax (Conrad), No. 142, and of 

 Atrypa spinosa Hall, No. 499 (Figs. 8 and 3). In tlie.se s])eci- 

 mens, however, the groove does not extend to the anterior ])or- 

 tion of the shell as it does in some. 



3. Rotation of the Umbos toward Each Other.— This rcsult.s- 

 in greater gibbosity of the shell. 



As shown above, the anterior growth of the valves in old age 



gerontic growth thus tends to ])usli the edges of the ephebic shell 

 farther and farther apart, and canses the valves to rotate outward 

 on the axis of tlie liinge Hue. This rotation l)rings the umbos 

 closer and closer togetlier until often tlie beak of the brachial 



valve encroaches on the dclthyrium of the pedicle valve to such 

 an extent as to block the original pedicle opening entirely (Figs. 



