534 Prof. Nicholson 8{ J. E. Marr — Phylogeny of the GrapfolUes. 



part of the polypary, the stipes curving backwards and then becoming 

 straighter in the distal portion ; while the hydrothec£e overlap, and 

 are variable in form. (See p. 531.) 



(3) Group containing Bryograptus ramosus, Brogger, Tetragrapfus 

 fruticosus, Hall, and Lidymograptus MurcMsoni, Boeck. In this 

 group the angle of divergence is less than 45°, the stipes being 

 sub-parallel in the distal portion of their course; vv^hile the hydro- 

 thecas are of the straight type, with straight apertures, and overlap 

 one another. (See p. 530.) 



(4) Group containing Dichograptus ocionarius, Hall, Tetrngraptus 

 serra, Brongn. [=zT. hryonoides, Hall), and Didymograptus arcuatus, 

 HalL In this group the angle of divergence is over 180° ; the 

 stipes are curved, and the hydrothec^ overlap and have curved 

 apertures. (See p. 532.) 



Group 5. 



Dichograptus octohracMahis. 



(5) Group containing Bicliograptus octobracJiiatns, Hall, and D. 

 Sedgwtchii, Salt., with Tetragraptus IJeadi, Hall, and Didymograptus 

 V-Jracttis, Salt. In the forms of this group the stipes are very 

 similar in character, all giving indications of the broken V-curve; 

 while the hydrothecse have straight apertures. (See also p. 533.) 



(6) Group containing an unknown Dichograptus, Tetragraptus 

 Bigsbyi, Hall, and Didymograptus gihberulus, Nich. (See p. 535.) 



In this group the angle of divergence is over 180° ; the stipes 

 are broad and sub-parallel in the distal portion of the polypary ; 

 and the hydrothecffi are deep and curved, with curved, mucronate 

 apertures. (See Nicholson, Ann. and Mag. Nat. Hist. 1875, p. 271.) 



