No. 470] 



OLD AGE IN BRACHIOPODA 



113 



Nucleospira ventricosa Hall. — Senility in this very small, Low- 

 er Helderberg species is shown by a conspicuous roughness or 

 lamellosity of the concentric growth lines in the otherwise smooth 

 shell, a change in the angle of curvature, and the formation of 

 a reentrant groove at the cardinal angles, the point where senility 

 is first indicated. No shell sufficiently senile to have developed 

 a groove in the anterior portion was observed. 



Athijris spirijeroides (Eaton).— One specimen from the Hamil- 

 ton of Eighteen Mile Creek, New York, No. 498, was 22 mm. 

 long and 26 mm. wide when senescent characters first appeared. 

 After that it grew 10 mm. anteriorly measured on the curve of 

 the shell. This growth increased the antero-posterior axis only 

 6 mm. The maximum width of the pedicle valve was increased 

 only 2 mm. though the total amount added to the width in old 

 age measured over the curve of the valves, was 11 mm. The 

 rest of the growth both anterior and lateral merely added to the 

 thickness of the shell. In this shell old age is expressed by the 

 lamellosity of the growth lines and the change in the angle of 

 curvature. This latter character is more especially noticed at 

 the sides of the pedicle valve as this valvt> piK^l up growth lines 

 here to a thickness of 4.5 mm. 'l'h{> lateral edges of the l)rachial 

 valve thickened less. A groove was fortned at the junction of 

 the valves. The cardinal angle enlarged from 100° in the mature 

 shell to 125° in paragerontism (Fig. 6). This specimen shows 

 the normal progression of old age characters from the cardinal 

 angles to the anterior border, in the first appearance there of the 

 lamellose growth lines and of the groove. This groove at the 

 death of the animal had advanced only halfwav to the anterior 

 border of the shell. A separate pedicle valve, Xo. {\'^:^, shows 

 the maximum thickening from each side of the muscular unpres- 

 sion to the cardinal angles, with the greatest thickening at the 

 edge of the valve. Figs. 5 and 6 show for comparison a mature 

 and a gerontic individual. 



Conclusions. 



jVIinot ('91, p. 151) says very suggestively: "I think it is now 

 conclusively established that there is in guinea pigs a progressive 



