78 GENESIS OF THE ARIETIDA. 
ragonal form, or rather an immature representation of it, occasionally occurred 
in some adult individuals of Agas. levigatum, in which the sides were flatter 
than usual. In Agas. striaries the quadragonal form and the siphonal line or 
keel were more decidedly expressed, as well as the tendency to elevate the abdo- 
men. In Agas. Sciyronianum after the earlier nealogic stages were passed which 
closely resembled the full grown of striaries, with the exception of the thicker 
pilz and somewhat deeper umbilicus, the adult showed a quadragonal whorl 
with a keeled abdomen and tuberculated pila. The old age had a smooth trigo- 
nal whorl. In Agas. Scipionis,’ which is a naturally distinct form, the extreme 
varieties had more involute whorls, smooth pile, and became trigonal and 
smooth at an early stage. 
Thus, at all stages of growth and decline, the correspondence or parallelism 
between the individual and the morphogeny of the series is complete. 
In the second subseries of Oxynoticeras we have found that there was one 
species, Oxyn. Lotharingum, in which the whorl during the last senile stage became 
completely rounded on the abdomen. ‘The sides became gibbous and narrower, 
thus showing a slight tendency to revert to the primitive form of the less dis- 
coidal Ayas. striaries and Psil. planorbe. These similarities were also greatly in- 
creased by the appearance of senile folds similar to the primitive pilations of this 
species and Psiloceras. The adults of the species of this subseries were also gera- 
tologous, in so far as the forms were not only much compressed and trigonal, but 
also smooth. The degeneration and the total loss of the hollow keel also oc- 
curred in this oldest stage. We should not be at all surprised if species should 
be found, and identified as belonging to this series, in which the hollow keel 
was either not present at any stage, or was only slightly indicated during the 
nealogic stages. hese adults would then correspond to the geratologous stage 
of Oxyn. Lotharingum, in the same way that Ast. Collenoti corresponded to the 
old of the normal forms of Ast. obtusum and _stellare. 
If a tendency to the inheritance of retrogressive characters be granted, 
and certainly their occurrence at earlier stages in successive species makes 
this view seem highly probable, then the same law of replacement which pro- - 
duced progression would now act upon successive organisms so as to produce 
retrogression. The observed phenomena indicate the direct replacement. of 
the characters of progressive ancestors by degenerate characters, which were 
first observable in the old age of these ancestors themselves. If there had 
been in most cases simply a mass of degenerate forms, without any defina- 
ble evidences of successive gradations, as in the famous instance of the Magnon 
examples of distorted Planorbide, it would be possible to say at once that 
the parallelisms of the geratologous period, with retrogressive characters in 
what we have called geratologous species of the same series, were purely homo- 
plastic correspondences. On the contrary, the gradations are perfectly well 
marked, as we have described them above and in the Introduction to this mono- 
graph, and the replacement of progressive characters by the geratologous takes 
place in strict accordance with the law of acceleration in heredity. 
+ Summ, Pl. xi. fig. 8: 
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