36 GENESIS OF THE ARIETIM1. 



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 Arietida3, the persistence of Nautiloidea until the 

 present time, and the absence of nostologic series in Nautiloids of the Mesozoic 

 and Neozoic, 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 

 Ammonitinae in his " Paleontologie 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 " periode embryonnaire," during 

 which it is smooth and the abdomen round ; the second period, or " premiere 

 periode d'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 

 " derniere periode 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 " premiere periode de degenerescence," during which the ribs or 

 tubercles begin to separate more widely and become depressed ; and the fifth 

 period, or " deuxieme periode de degenerescence," 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." x 



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. 



