No. 470] 



OLD AGE IN BRACHIOPODA 



97 



gerontic portion there are at least eight in a length of but 10 mm. 

 (Figs. 1 and 2.) 



A specimen of Atrijpa spinosa Hall, No. 499, has in its mature 

 or ephebic stage a length antero-posteriorly of 30 mm. or of 36 

 mm. measured on the curve of the pedicle valve. In succeeding 

 growth originated a change in the angle of curvature at the an- 

 terior portion of the shell, indicating old age (sec below ). l^om 

 the umbo up to and including the mature portion nt' the slicll 

 there are 24 well marked growth lines about c(|ui(li>tant : on the 

 gerontic or deflected portion there arc nine uroutli lines in a 

 space of but 4.5 mm. Thus in ohl age the -lowtli lines become 

 crowded, as one occurs in every 0.') nun., while in the previous 

 growth one occurred in only every 1..") nun. '1 he ])iling iij) of 

 the growth lines was caused by their continued ]>i-()(lucti()n iniac- 



