116 



THE AMERICAN NATURALIST [Vol. XL 



s])iriJeroides, No. 498 (Fig. 6), and Atrypa spinosa, No. 499 (Fig. 

 3), as described above, it is evident that the growth of the soft 

 parts must have practically ceased, while their secreting activities 

 were continued, but were now almost entirely directed toward 

 thickening the shell (see also Beecher, :01, p. 91). 



The formation of a groove at the junction of the valves means 

 further, not only that the growth of the mantle has ceased, but 

 that it is in fact growing smaller, in other words is shrinking. As 

 noted above in the description of Athyris spiriferoidcs, etc., the 

 width of the shell on the right and left axis is less during parageron- 

 tism than it is during the earlier anagerontic stage. This tendency 

 of the soft parts of the animal to shrink and to express this shrink- 

 age in the hard parts is also well exhibited among pelecypods and 

 cephalopods. 



The lamellosity of the growth lines in such types as Athyris, 



Atrypa, etc., is caused by the relatively decreasing extent of the 

 successive lamella> of shell growth built in old age. This as result- 

 ant brings about a change in the angle of curvature which in 

 extreme cases causes even a resultant angle of less than 90°. 



The thickening of the shell on the interior often takes place 

 very irregularly, and leaves the surface strongly papillose. This 

 is well seen in the Spirifers, Stropheodontas, etc. In some Strophe- 



