"Prof. Nicholson 8^ J. E. Marr — Phytogeny of the Graptolites. 633 



the case of the "genera" DicJiograptus and Bryograptus, we con- 

 fidently predict the discovery of forms of these or allied " genera " 

 which will agree with the four remaining Tetragrapti as regards the 

 above-mentioned characters. 



Taking the series shown in our illustrations in order, we find the 

 following points of agreement in the members of each group of the 

 series, the chief or only points of difference being in the number of 

 stipes in the polypary. 



Guorp 7. Group 5. 



Tetragraptus Tetragraptus 



crucifer Headi 



Didymograptus Didymograptus 



pennatulus 



V- f ractus 



(1) Group containing Bryograptus Gallavei, Lapw.. Tetragraptus 

 Sichsii, Lapw. and Hopk., and Didymograptus affinis, Nich. In this 

 group the stipes are narrow, and the hydrothecas are of the rect- 

 angular straight type, and are in contact only or overlap very slightly ; 

 while the angle of divergence is about 90°. (This latter feature is 

 not quite clear in the case of Tetragraptus Hicksii.) (See p. 580.) 



(2) Group containing Bryograptus retroflexus, Brogger, Tetra- 

 graptus denticulatus, Hall, and Didymograptus fasciculatus, Nich. In 

 this group the angle of divergence is nearly 180° in the proximal 



