102 



THE AMERICAN NATURALIST [Vol. XL 



! species the median sinus appears to become more ac- 

 with age. For example, Athyris spiriferoides shows 

 well this accentuation of the sinus with hardly any corresponding 

 development of the median 

 fold, while in Ccelospira (jra- 

 baui Shimer, both sinus and 

 fold are developed (Shimer, 



^£utriJL7oZsz::.\:::'i:^« --oi.?- 253). 



Helderberg of Clarksberg, New York. No. 6. Enlargement of Cardi- 



4, Harvard. Angles. — The cardinal 



angles, that is, the angles made at the cardinal extremities 

 between the hinge line and the sides of the shell, enlarge during 

 senescence. 



Examples: a specimen of Rafinesquina alternata, No. 128, has 

 just preceding senescence, a cardinal angle of 87°. This increased 

 to 99° during old age (Fig. 13). 



A specimen of Spirifer mucronatus var. thedfordense, No. 405, 

 has at the close of the neanic or Spirifer mucronatus stage (Shimer 

 and, Grabau, :02, p. 171) a cardinal angle of 25°. This angle rap- 

 idly increases as seen in Fig. 16, through the ephebic and gerontic 

 stages until it measures 60° at the death of the animal. 



7. Reduction of Shell Index.— The shell 

 index, i. e., the breadth divided by the 

 length, becomes smaller with old age (see 

 Cumings, :03, p. 3). In other words the 



shell becomes proportionally longer in old yig. 16.— a senile form of 

 age than in maturity and in_ this respect ^S/™™™? Ta 



approaches the nepiomc condition. Grabau, from the Hamil- 



Examples: a specimen of Spirifer mucro- xhis^shmts seniiiu-'in^t^ 

 natus var. thedfordense has during its enlarging' (anUnai aiigi.s 

 nepionic stage a shell index of 1.77; dur- \^^^.^ xo. 405. iiai vani. 

 ing its neanic, 3.57; and during ephebic, 



1.90. For further measurements and discussion of the varietal 

 form see Shimer and Grabau (:02, p. 174). 



An old specimen of the above species, No. 405a, had during 

 early maturity a width of 34 mm. and a length of 14 mm., giving 

 a shell index of 2.43. In old age the width was 33 mm., the length 

 18 mm., and shell index 1.S3. 



