200 A. HYATT ON THE PARALLELISM BETWEEN THE INDIVIDUAL 



species and, therefore, vastly surpass their predecessors the discoidal Nautili, which are rep- 

 resented by only a few more than twenty species in the same formation. The aberrant Am- 

 monoids differ from their prototypes in one respect : they do not outnumber the discoidal 

 shells in the Cretaceous, as the aberrant Nautiloids of the Silurian did the normal forms of 

 their special group, but the normal Ammonites have much the largest number, being as 

 one hundred and forty-four to eighty-six. In the last horizon of the Cretaceous, it is true 

 that the polar forms exceed them, as has been shown, by about eight species, but this is only 

 sufficient to demonstrate that the latter are gaining, and does not afford margin enough for 

 the assumption that their superiority is comparable with the vast predominance of the 

 aberrant Nautiloids in the Silurian. Although the polarity is here less accurate than in 

 other respects, the succession of the four epochs resembles the succession of the four peri- 

 ods of the individual both in number and their general structural peculiarities. 



The first epoch of the order is especially the era of rounded, and, in the majority of the 

 species, unornamented shells with simple septa ; the second is the era of ornamentation, and 

 the septa are steadily complicating ; in the third the complication of the septa, the orna- 

 mentation, and the number of species, about twice that of any other epoch, all combine to 

 make it the zenith of development in the order ; the fourth is distinguishable from all the 

 preceding as the era of retrogression in the form and partially in the septa. 



The four periods of the individual are similarly arranged and have comparable charac- 

 teristics. As has been previously stated, the first is smooth and rounded with simple septa ; 

 the second tuberculated and the septa more complicated ; the third was the only one in 

 which the septa, form, and ornamentation simultaneously attained the climax of individual 

 complication ; the fourth, when amounting to anything more important than the loss of 

 a few ornaments, was marked by the retrogression of the whorl to a more tubular aspect, 

 and by the partial degradation of the septa. 



This exhausts the correlations between the individual as a unit and the whole order. 

 There yet remain, however, those which may be traced in the proportions of the embry- 

 onic, adult, and old age features of the shell ; by their aid we shall be able to perceive 

 the extraordinary correspondence of the life of the individual with its position in the struc- 

 tural scale. 



The adult period of the individual is evidently that which differs most from all the rest, 

 since these, as has been shown, are inversely repeated on either side, and it alone remains 

 non-conformable to any other. 



Examining the order, it is not difficult to perceive that at the epoch of the Jura, corre- 

 sponding to the adult period of the individual, the Ammonites depart to the utmost extent 

 from their polar types the Orthoceratite and the Baculite, since, as we have also seen, this 

 epoch is remarkable for the combination of all the elements of complication and their 

 simultaneous arrival at the maximum of vital intensity. 



It is also most remarkable, that the growth of the individual itself is strictly conform- 

 able with this law, being more homogeneous or embryonic in the Orthoceratite, heterogene- 

 ous or continuing to increase the diversity of the adult from its own young throughout life 

 at the era of greatest vital intensity among the Ammonites, and more homogeneous or em- 

 bryonic again in the Baculite. An adult Orthoceras has precisely the same concave septa 

 and round whorl as the young ; the septa of an Ammonite may vary among the keeled 

 group from four rounded to eighteen foliated lobes, and the form from an open coil to a 

 completely covered umbilicus (Amm. discoides Ziet., Amm. discus Sow.), or among the smooth 



