588 



NEW YORK STATE MUSEUM 



The application of the results of Jack- 

 son's investigations to the colony of Gonio- 

 graptus is fruitful in more than one regard. 

 It permits us to conclude that the branches 

 of the hydrosome, 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 tubu- 

 lar thecae, with less overlap, looser ar- 

 rangement, smaller deviation from the di- 

 rection of the axis of the branch and 

 straight, not mucronate apertures. Like- 

 wise, the whole colony was derived from 

 colonies composed of such thecae, which are 

 still retained in its oldest parts. The Cam- 

 bric species of Bryograptus and Clonograp- 

 tus exhibit well these types of thecal ar- 

 rangement and structure. In the genera 

 Tetragraptus, Didymograptus and Phyllo- 

 graptus, where, within the Dichograptidae,, 

 the thecae have advanced farthest beyond 

 their original form, the process of ontoge- 

 netic acceleration has also gone farthest in 

 . effacing all vestiges of the original thecal 

 form, as e. gr. in Didymograptus (I s o- 

 graptus) gibberulus, where no sicoi- 

 dal thecae are preserved. In others, how- 

 ever, as a study of Hall's excellent 

 figures of the various species of Didy- 

 mograptus will show, the gradual 

 change from tubular to more gibbous, 

 more closely arranged and more erect 

 thecae can clearly be traced. The writer 

 fruticoL s Te H t aii asr lr P a t Bch desires to illustrate these ontogenetic 



which shows progressive , . , . , » 



change of thecae. x2H changes in the stages and arrangement 01 



