532 NEW YORK STATE MUSEUIM 



within these branches also a gradual change of the thecae from the stolonal 

 or siculoid form to the brachial one takes place. In the species of Gonio- 

 graptus cited, it has been shown that the earlier thecae of the branches 

 have an inclination of but 7° as against 28° in the more distal, mature 

 thecae, and overlap ])ut one fourth of their length, whereas the later ones 

 overlap more than one half and have straight apertures without marginal 

 processes ; these also have concave apertures with projecting, outer apertural 

 margins. 



It has been concluded from these facts that " the thecae of the colony of 

 Goniograptus (and other Dichograptidae as well) from the sicula through the 

 stolonal and early brachial thecae to the distal brachial thecae, form an 

 ontogenetic series, which furnishes a clear and interesting example of 

 localized stages of development," the existence of which has been demon- 

 strated and their character elucidated by R. T. Jackson^. The principle of 

 the localization of development expresses the fact that " in organisms that 

 grow by a serial repetition of parts, it is found that there is often an 

 ontogenesis of such parts which is more or less clearly parallel to the 

 ontogenesis of the organism as a whole." 



The asexually produced buds of the graptolites (thecae), like those of 

 Hydrozoa and Actinozoa, are now to be considered as such localized stages. 

 While they lack the stages seen in early embryonic development, they 

 repass in general the later stages found in the ontogeny of sexually produced 

 young. 



The application of Jackson's principle to the colony of Goniograptus 

 hence allows the conclusion " that the branches of the rhabdosome, like the 

 leaves of a tree, indicate individually by their ontogeny the path along which 

 they have been developed. The ontogeny of the branches demonstrates 

 that the phylogenetically preceding forms possessed branches composed of 

 more tubular thecae, with less overlap, looser arrangement, smaller deviation 

 from the direction of the axis of the branch and straight, not mucronate 

 apertures." 



1 Bost. Soc. Nat. Hist. Memoir. 1899. v. 5, no.4. 



