36 GENESIS OF THE ARIETID/. 
Ascoceratites, Discoceratites, and Ophidioceratites also indicate lost faunas, in 
which such types had their now unknown, but progressive progenitors. 
Notwithstanding, however, these geratologous series and the facts already 
stated, the shells of the surviving stocks of Nautiloidea were not usually so per- 
ceptibly changed in old age as the more specialized shells of Ammonoidea. So 
far as known, the shell of a nautiloid, whether fossil or recent, though it may 
lose tubercles and perhaps become somewhat changed, neither becomes very 
decidedly depressed nor decreases perceptibly in the involution of its whorls 
during old age. This remarkable exhibition of persistent growth force in indi- 
viduals, when taken in conjunction with the slight senile metamorphoses of the 
smooth radical types of the Ammonoidea and the persistence of the keel in the 
normal progressive forms of the Arietids, the persistence of Nautiloidea until the 
present time, and the absence of nostologic series in Nautiloids of the Mesozoic 
and Neozoie, all show how complete is the correspondence between the ontogeny 
of individuals and the phylogeny of the groups to which they belong. 
Alcide d’Orbigny drew attention to the old age of the individual among 
Ammonitine in his “ Paléontologie Francaise.” He divided the life of the indi- 
vidual into five periods, distinguishable from each other by the external character- 
istics of the shell; namely, the first period, or “ période embryonnaire,” during 
which it is smooth and the abdomen round; the second period, or “ premiére 
période @accroissement,” which is marked by the advent of the tubercles, or ribs 
and keel, if there are to. be any upon the adult shell; the third period, or 
“derniére période d’accroissement,” during which the tubercles or ribs and the 
keel are fully developed, and the whorl takes on its adult configuration ; the 
fourth period, or “ premiére période de dégénérescence,” during which the ribs or 
tubercles begin to separate more widely and become depressed ; and the fifth 
period, or “ deuxiéme période de dégénérescence,” when all these ornaments are 
obsolete, and the exterior is smooth again as in the young. 
The recapitulation in which he sums up the results of this remarkable series 
of observations is equally truthful and instructive. The following paragraph 
conveys the sense of the original, though its piquancy and force is lost in trans- 
lation. “These modifications are not due to chance, but to decided regularly 
occurring periodical metamorphoses, which affect the larger number of the 
Ammonites, and which invariably operate in a regular order of succession. In 
fact, each one, though smooth in the youngest period, covers itself at a later time 
in the course of growth with tubercles around the umbilicus, afterward with ribs, 
striations, or tubercles upon the back (abdomen). It is then in the adult stage. 
Having arrived at the maximum of external complication, all of these ornaments 
begin to show signs of alteration ; it (the shell) degenerates; its striations and 
dorsal (abdominal) ribs first disappear; then follow its lateral ribs or tubercles, 
and in old age it becomes fully as simple externally as it was during the em- 
bryonic period.” * 
1 There are apparent exceptions to this law, as observed above, in the heavy folds of the senile stages 
of many forms in the Upper Jura, and some in the Lower Jura. The young of these forms, which have not 
yet been investigated closely, ‘will, however, probably explain this discrepancy by showing that the senile 
folds correspond with larval folds, as is the case in Oxynoticeras of the Lias. 
