No. 470] 



OLD AGE IN BR AC H 10 POD A 



119 



stage, yet it is a resemblance due to loss of characters. It is thus 

 essentially different from the developing of the similar characters 

 in youth. The characters usually disappear from the shell in the 

 inverse order of their initiation (see also Hyatt, '94, p. 20, and 

 Beecher, :01, p. 2G9). 



As seen above, senility is first shown at the cardinal angles 

 and from there it takes place progressively to the anterior portion 

 of the shell. Hence it is at the cardinal angles that we look for 

 the first expression of old age, — as a change in the angle of cur- 

 vature, lamellose growth lines, flattening of ribs, and development 

 of a groove at the junction of the valves, ^'ery rarely are indi- 

 viduals found sufficiently old to have expressed on the anterior 

 portion of the shell all of the above senile characters. 



When these characters do not appear simultaneously on the 

 shell they appear in a definite order, viz., (1) flattening of ribs, 

 (2) lamellose development of concentric growth lines, (3) change 

 in the angle of curvature, (4) formation of a groove at the junc- 

 tion of the valves, (5) flattening of sinus and fold. This is the 

 usual order, though at the cardinal angles they frequently occur 

 at approximately the same growth line. 



Originating thus at the cardinal angles, these gerontic features 

 are pushed farther and farther forw^ard until in paragerontism 

 they are present on the most anterior portion of the shell. 



Geological Departmext 



