Trof. Nicholson 8^ J. E. Marr—PhijJogemj of the Graptolites. 585 



(7) Group containing an unknown Dirhogr aphis, Tetragr aphis 

 crucifer, Hall, and Didymograptus pennahihis, Hall. (See p. 533.) 



Jn this group the angle of divergence is at first less than 180°, 

 but the stipes (in each pair of the Tetragraphis) are finally placed in 

 the same straight line. The stipes are broad, and the hydrothecee 

 are inclined at a high angle to the line of the back of the stipe, 

 and overlap one another. 



(8) Group containing an unknown Dicliograptus, Teh-agmphis 

 quadrihracliiahis. Hall, and Didymograptus extensus. Hall. (See 

 p. 536.) In this group the angle of divergence is under 180°, the 

 stipes (of each pair in the Teh-agraphis) are in the same straight line. 

 The hydrothecse are of the straight type, with straight cell-apertures. 



(9) Group containing an unknown J)ichographis, Tetragraphis 

 appro ximntus, Nich., Siwd Didymograptus nitidus, Hall. (See p. 537.) 

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

 curved in their proximal portion and afterwards placed in the same 

 straight line (each pair in the case of the Titragraptus). The stipes 

 are narrow, and the hydrothecaj are of the straight type, with straight 

 apertui'es. 



Group 6. Group 6. 



Tetragraptus . Didymograptus 



Big-sbyi g-ibberulus 



It must not be supposed that in all of the above-mentioned groups 

 the nearest four-branched form to the corresponding eight-branched 

 one, or the nearest two-branched form to the corresponding tour- 

 branched one, has been actually selected. For instance, in the case 

 of the second group, the hydrothecae of Didymograptus fasciculatus, 

 Nich., are very different in some respects from those of Tetragraptus 

 denticulatus, Hall, and Bryograptus retroflexus, Brogger. We believe 

 however, that in such cases other forms will ultimately be lound 

 which will remove this discrepancy. It is probable that a whole 

 group of species sprang from an earlier form which possessed a greater 

 number of branches than its descendants. Thus, the whole series of 

 the" tuning-fork " Didymograpti— the Murchisom group— is generally 

 admitted to form one group, and we believe that all the species 

 included in that group have sprung from Tetragraptus fruticosus, 

 Hall. Many of the species of this group of Didymograpti agree 

 much more closely with Tetragraptus fruticosus as regards the 



