588 



XBW YORK STATE MUSEUM 



The ai»i)Ii(*ation of the results of Jark- 

 son's invest ijjations to the eolony of Goni«>- 

 «;:rai»tiis is fruitful iu more than one regard. 

 It permits us to conclude that the branches 

 of the hydrosonie. 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 jihylogenetically 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 thmae, which are 

 still retained iu its oldest parts. The Cam- 

 bric species of Bryograptus and Clonograp- 

 tus exhibit well these tyiK^s 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. g. in 1) i d y m o g r a p t u s (Is o- 

 graptus) gibberulus, where no sicoi- 

 dal thecae are preserved. In others, how- 

 ever, as a study of Uall's excellent 

 figures of the various species of I>idy- 

 mograptus will show. the gradual 

 change from tubular to more gibbous, 

 more clos(*ly arranged and more erect 

 thecae can clearly be traced. The writer 

 Fig 15 Tetragraptus dcsircs to jllustratc thcsc ontogcuetic 



frutlcosus Ball. Branch '^ 



c£inge of'th^e.^'^hT**''* chaugcs iu the stages and arrangement of 



