THE THREE MODES OF DEVELOPMENT. 37 
The accomplished author of the “ Paléontologie Francaise” denied that the 
internal parts were affected by old age, “ne montrent qu’une complication 
’ This error was corrected by 
Quenstedt who pointed out that the closer approximation of the sutures in large 
individuals was due to senility, and the author is now able to record that he has 
either observed or seen figured similar cases of approximate sutures, indicating 
senile degradation in all of the different forms of chambered shells, except in 
Belemnoids, which have not yet been investigated. 
D’Orbigny and Quenstedt were both satisfied with noting the details of 
the old stage in the individual, the diseased aspect of certain forms, and their 
reproduction of the characters of their own young and of those of older forms, 
but did not attempt to explain the wider meaning of these parallelisms. In 
former papers we have asserted that the close similarity between the smooth, 
straight Baculites of the Cretaceous, the extreme nostologic form of the Ammo- 
noids, and the smooth, straight Orthoceras of the Cambrian, the common radical 
from which both Ammonoids and Nautiloids sprang, is parallel with the resem- 
blances which exist between the nostologic or oldest stages of the individual and 
its own young. 
This resemblance between radical and geratologous forms in smaller groups, 
like the Arietide, was slighter, and often consisted merely in the smoothness 
of the shell, or loss of the keel, or decrease in the amount of involution of 
the whorl. As?. Collenoti and Psil. planorbe both have the compressed helmet- 
shaped outline of the whorl in section, and are smooth, though Collenoti exag- 
gerates this form, or is more involute and flatter than planorbe. The most 
remarkable cases of geratologous reversion in the Lias are found in Ozynoticeras 
Lotharingum, in which the old whorl loses its keel, and exactly reassumes through 
degeneration the compressed helmet-shaped aspect of the adults of Psd. planorbe. 
Iiven this extreme example among Arietide, however, is not in any sense 
an uncoiled shell. It is very nearly a complete parallel with the smooth 
keelless proximate radical form Psiloceras, and may therefore be termed a noso- 
logic species. It barely attains this extreme rank in degeneration, whereas other 
toujours croissante et jamais de dégénérescence.’ 
species, such as As?. Oollenoti, which retain the keel and do not decrease in size of 
whorl, are only clinologic approximations. 
The resemblances which occur between the young and old of the same indi- 
vidual in the same parts and organs take place because the organs lose their 
power to exercise the functions which distinguished them in the adult, and 
becoming useless, are either partly or wholly atrophied and resorbed. 
Tur TuHree Moprs or DEVELOPMENT. 
The likeness between the younger stages of growth and the senile stages 
of decline in the same individual is, as we have just shown, due to the disap- 
pearance in old age of the specialized ephebolic characteristics acquired in the 
progressive nealogic and adult stages of growth. In groups the resemblances 
between radical and geratologous forms is occasioned by a similar suppression in 
