GRAPTOLITES OF NEW YORK, PART 1 643 



Elles and Wood. These authors have divided the species according to the 

 ultimate direction of their branches into series groups, some of which fall 

 again into groups by the character of their thecae. In applying to our 

 species of Tetragraptus this system, which, according to present knowledge, 

 unites the members of phylogenetic series we obtain the following series 

 and groups. 



Horizontal series Group 1 .... Type T. quadribrachiatus 



T. quadribrachiatus 

 T . am ii 



Dependent series Group 2 Type T. fruticosus 



T. fruticosus 

 T. e larkei 



Group 3 Typo T. pendens 



T. pendens 



Reclined series Group 4 Type T . s e r r a 



T . serra 



Group 5 Type T . s i m i 1 i s 



T . s i in i 1 i s 



T. w o odi 



T. pygmaeus 



Group 6 Type T. taraxacum 



T . ta r axacu m 



Flexuous (Etagraptus) series Group 7 Type T . 1 e n t u s 



T. lentus 



Holm has succeeded in isolating the rhabdosomes of a species of 

 Tetragraptus ( T . s i m i 1 i s ) and thus demonstrated that from the sicula 

 near its apex originates a first theca, from which a second theca buds that 

 turns to the other side. The development up to this stage is exactly 

 homologous to that in Didymograptus. In each of the two diverging thecae 

 a dividing wall appears, so that from each of these thecae two new 

 thecae are originated. Each of the four resultin£; thecae becomes the mother 

 theca of a branch. We have copied some of Holm's excellent drawings which 

 illustrate this development [text fig.48-50]. 



