5W Prof. Nicholson 8f J. E. Ilarr — PJiylogeny of the GraptoUtes. 



cliai'acters of the hydrothecEe than does D. MurcMsoni, and we have 

 figured this latter simply because it is so well known. Again, it is 

 quite conceivable that a DiSymogra'ptus which from its general shape 

 would be placed in the MurcMsoni group, might show hydrothec^ 

 entirely different in character from those of Tetragraptus fruticosus, 

 and might thus be shown to have descended from an altogether 

 different type of Tetragraptus. That the Didymograpti were divisible 

 into groups has long been known ; and in 1885 Otto Hermann 

 suggested (Die Graptolithenf'amilie Dichograptidge, Lapw.) six such 

 groups, the types of these being respectively Didymograptus patidns, 



Group 8. 



Tetragraptus quadribrachiatus 



€:>^>^'^>:^>^^^>^>^ 



Didymograptus extensus 



Hall, D. MurcMsoni, Boeck, J), fasciculatus, Nich., J), pennatulvs, 

 Hall, D. affinis, Nich., and B. V-fractus, Salt, Some of these 

 groups, as before remarked in the case of the MurcMsoni series, 

 are certainly natural, while others will most likely be ultimately 

 re-divided and will undergo a redistribution of their contained 

 species. Each group, however, will probably be found to present 

 close affinities to one particular species of Tetragraptus. 



It is very difficult to understand how the extraordinary resem- 

 blances between the various species of Tetragraptus and Didymo- 

 graptus have arisen, if, as usually supposed, all the species of these 



