532 Prof. Nicholson 8f J. E. Marr — Phylogeny of the GraptoUtes. 



In our illustrations we show nine species of Tetragraptus ; and we 

 are not acquainted with any others which have with certainty been 

 referred to this genus, except T. alatus, Hall, which is only known 

 from examples in very bad preservation. Eight of the Tetragrapti 

 are represented by forms of Bidymograptus which are closely com- 

 parable with them as regards the characters of the hydrothecse 

 and the amount of the angle of divergence ; whilst the ninth 

 (Tetragraptus denticulatus, Hall) is comparable with Didymograptus 



Group 4. 



DiCHOQRAPTUS octonarms 



Tetragraptus serra 



Didymograptus arcuat-us 



fasctculatus, Nich., as regards the angle of divergence, though the 

 characters of the hydrothecee of the two differ considerably in 

 minor points. Moreover, four of these four-branched Tetragrapti 

 are represented, as regards the characters of the hydi-othec^ and 

 the amount of the angle of divergence, by forms of DicTiograptus 

 or Bryograptus, with eight or more branches, and a fifth by a 

 Bryograptus which agrees in the amount of the angle of divergence, 

 though the hydrothecse differ in minor points. Considering the 

 amount of unworked material which we know to be in existence in 



