SUPPLEMENT.— AMMONITE DEVELOPMENT. cciii 



Numerically, anagenesis musters more than catagenesis: bu1 the features which 

 it affects are less important, and to the latter must be credited bulk-decline — a 

 factor in which is whorl compression. For even if the same diameter should be 

 attained, it is only gained by a great decrease in thickness. 



It is now necessary to show what bearing these considerations have on the 

 interpretation of the generic definitions and the specific descriptions. 



The definition of the genus indicates the characters shown by a certain species, 

 and by a particular specimen of that species, which is selected as the type of a 

 certain genetic series. This type-species of the genus — the genotype — then becomes 

 a kind of fixed point in the genetic line. The species leading up to the genotype 

 — in a catagenetic series like the Hildoceratidaa — will show in the main more, and 

 those leading from it less, accentuated characters than the genotype. 1 The onto- 

 geny of the genotype, and of species of allied genera, will give evidence as to the 

 course of development. If the genotype show a tuberculate stage in youth 

 declining to costate and smooth stages in adult — expressed in the specific descrip- 

 tion, which must be read with the generic definition, as tuberculate, to costate, to 

 levigate — then the species leading up to the genotype should show more of the 

 tuberculate and less of the subsequent stages, while the species leading from the 

 type should show less of the tuberculate and constantly more of the subsequent 

 stages (compare Sonninia and the Hildoceratidge, passim). If the type show 

 inflated whorls in youth, becoming compressed in adult, the species leading up to 

 it should show a longer persistence of inflated whorls, and little or none of the 

 compression, the species leading from it constantly less of the inflated period and 

 more of the compression. And so with other features. These are the phenomena 

 of morphic prefiguration and representation (p. 315). Under circumstances 

 such as these the specific descriptions may often seem to contradict the generic 

 definitions ; but they do not : they merely indicate the difference in the degree 

 of development between the species and the type. The genotype of series 

 X may be defined as sublatumbilicate, that of Y as angustumbilicate ; but a smooth 

 species assigned to X may be described as angustumbilicate, indicating the change 

 in the series ; but the smooth species of Y would probably by then be perangust- 

 umbilicate unless excentrumbilication had commenced. 



If, however, the definition of the genotype of X be sublatumbilicate while a 

 species assigned thereto be stated as latumbilicate, it indicates that such species is 

 in an earlier stage of development, it may be ontogenetic, it may be phylogenetic, 

 but it will be found to be the morphic prefiguration of a younger stage in the ontogeny 

 of the genotype. In the specific descriptions some such correlation as this would 

 be found : X 1 spinous to costate, latumbilicate, X 2 (the genotype) costate to levi- 



1 The opposite obtains in anagenesis. Compare Arnioceras. 



