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When we attempt to understand these pathologic uncoiled series 
and forms, which show by their close-coiled young that they were 
descended from close-coiled shells, we find ourselves without com- 
parisons or standards in the early life of the individual. The laws 
of geratology—that the old age of the individual shows degradation 
in the same direction as, and with similar changes to those which 
take place in successive species or groups of any affiliated series of 
uncoiled and degraded forms—here come into use, and serve to ex- 
plain the phenomena. ‘This correspondence is shown in the uncoil- 
ing of the whorls, loss of size, the succession in which the orna- 
ments and parts are resorbed or lost, the approximations of the 
septa, and position of the siphuncle. It is quite true, as first stated 
by Quenstedt and also by D’Orbigny, that every shell, when out- 
grown, shows its approaching death in the closer approximation of 
the last sutures, the smoothness of the shell, the decrease in size, 
~etc.; but, in order to realize that these transformations mean the 
same thing as those which take place in any series of truly retro- 
gressive forms, we have to return to the types in which unfavorable 
surroundings have produced distortions or effects akin to what 
physicians would term pathological. 
This frequently happens in small series of Nautiloidea; and, if 
we confine ourselves to these, we can make very accurate com- 
parisons: or, on the other hand, in the case of the Ammonoidea, 
we may trace the death of an entire order, and show that it takes 
place in accordance with the laws of geratology. Such series, 
among the Nautiloidea, are abundant in the earlier formations ; but 
they have not the general significance of the similar forms among 
the Ammonoidea, and can be neglected in this article. There are 
no known cases of degraded series of uncoiled forms among the 
ammonoids of the earlier or Paleozoic periods ; they may have oc- 
curred, but they must have been excessively rare. 
In the Trias and early Jura, pathologic uncoiled forms are rare 
among ammonoids, but in the Middle and Upper Jura they increase 
largely ; and finally, in the Upper Cretaceous they outnumber the 
normal involute shells, and the whole order ceases to exist. Neu- 
mayer has shown, that a similar degradation occurs in all of the 
normal ammonoids of the Cretaceous, and that their sutures are less 
complicated than those of their immediate ancestors in the Jura. 
This proves conclusively, that the degeneration was general, and 
affected all forms of Ammonoidea at this time; since the uncoiled 
