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THEORY OF RADICALS AND MORPHOLOGICAL EQUIVALENCE. 55 
Degradation in the ornaments, markings, etc., occurred, but is less marked 
and rarer on account of the frequent absence of the shell. Prof. James Hall 
has figured cases of senile degradation! in Orthoceras, and we have ourselves 
seen several similar examples. 
We have not been able to trace any remarkable changes in old age among 
the silurian, devonian, or carboniferous goniatitine. ‘The dyassic and _ triassic 
forms of Ammonoidea with highly ornamented shells have not, as far as known, 
exhibited cases of senile metamorphosis in any noticeable abundance, and there 
is a marked absence of these in Mojsisovies’s plates, although a few are figured. 
There is an easily observed increase in the effects of old age upon. the last 
whorls of the shell in the Jura. Every group, however, does not show the effects 
of senility equally. There are not only less remarkable metamorphoses in the 
radical genus Psiloceras, but also less in the Arietide, as a whole, than in the Am- 
monitinee of the Upper Jura. This retrogression’ correlates directly with the 
increasing prevalence of geratologous uncoiled shells in the Cretaceous. There 
is, therefore, among Ammonoidea a general progress up to the Jura, which is 
definitely expressed in the life of the individual as well as in the life of the 
type, and a general decline in the later Jura and Cretaceous, which is also defi- 
nitely expressed in a similar way. Ge ratologous types and forms are also less 
frequent among the paleozoic and earlier mesozoie than in later mesozoic series. 
Individuals apparently had greater strength as individuals in these earlier periods, senile 
melamorphoses being less marked in their old age. The phenomena presented by 
radical types also accord with this statement. If we pick out those types which 
were the progenitors of series, they appear to have been less affected by degra- 
dational changes than the more specialized forms which arose from them. This 
fact, however, as we have often stated, corresponds directly with the more com- 
plicated organization of derivative forms, as contrasted with the simpler structures 
of radical forms. There are more characters introduced in the adults of special- 
ized derivatives, and the necessary disappearance and degradation of these marks 
the old age of the individual in such types with more obvious modifications. 
As we have stated above, however, geratologous metamorphoses do occur 
even in Orthoceratites, and series of Nautiloidea. The Lituites of the Phillips- 
burg (Canada) and Fort Cassin limestones,? which we are now studying, and the 
Lituites and Trocholites described by Holm,’ have in their youngest stages forms 
which indicate derivation from nautilian shells, thus proving that they are not 
radical forms, but degenerate uncoiled derivatives of prepaleozoic or paleozoic 
stocks of close-coiled Nautiloids, of which they are the last survivors. Trocho- 
ceran species belong to several different genera, and are all degenerate forms. 
? Orthoe. fusiforme, figured in Nat. Hist. of New York, Paleont., I., pl. xx. fig. 1, is a very large specimen, 
with the last three sutures nearer together than the preceding, and this generally indicates advanced age 
among Ammonitine. The increasing width between the folds in the shell of Orth. crotalum, Hall, Paleont., 
V., pt. 2; pl: ali. fig. 1, 9, 6, though characteristic of the living chamber, as described by him, probably 
became permanently characteristic of the senile stages. Very large specimens of Endoceras not infrequently 
show approximation of the sutures and less distinct annulations than in the adult stage, though this does not 
appear to be an invariable accompaniment of age, as in the Ammonitine., 
? Whitfield, Bull. Am. Museum, New York, I., No. 8. 
8 Dames et Kayser, Pal. Abh., III., pl. i. and y. 
