THEORY OF EADICALS AND MORPHOLOGICAL EQUIVALENCE. 35 



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 1 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 goniatitinas. 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 Mojsisovics'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 Arietidaa, as a whole, than in the Am- 

 monitinse 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. Geratologous types and forms are also less 

 frequent among the paleozoic and earlier mesozoic than in later mesozoic series. 

 Individuals apparently had greater strength as individuals in these earlier periods, senile 

 metamorphoses 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, 2 which we are now studying, and the 

 Lituites and Trocholites described by Holm, 3 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. 



1 Orihoc. fusiforme, figured in Nat. Hist, of New York, Paleont., I., pi. 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 Ammonitinae. ' The increasing width between the folds in the shell of Orth. crolalum. Flail, Paleont., 

 V., pt. 2, pi. xliii. fig. 1, 2, 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 Ammonitinse. 



2 Whitfield, Bull. Am. Museum, New York,' I., No. 8. 



3 Dames et Kayser, Pal. Abh., III., pi. i. and v. 



