568 NEW YORK STATE MUSEUM 



dichograptid graptolite stock. The latter fact finds its 

 strongest expression in the remarkable restriction in the num- 

 ber of species of dichograptids, but is also recognizable in the 

 character of the species remaining. Of the multiramous 

 dichograptids only Goniograptus is left, which, as will be 

 shown in another article, has here become so fixed and un- 

 changeable in the number of its branches that it is clearly a 

 final derivative of the freely branching older Clonograptus 

 forms. The species of the genus Tetragraptus, which 

 latter reached the acme of its development in the preceding 

 zone, are, with the exception of T. quadribrachiatus, 

 crucifer and a 1 a t u s , still present, but are represented 

 only by smaller mutations. The genus Didymograptus, which 

 lived into the Trenton period, and with a few species of 

 its subgenus Leptograptus even extends into the Utica 

 period, appears with two new species, which are restricted to 

 this zone, and one of which, Didymograptus bifidus, 

 is in the Deep kill section the most characteristic species of 

 the zone, as it is extremely common in the beds, extends 

 neither above nor below the zone and is the first of the " tuning 

 fork " species of the genus, which in Europe are quite char- 

 acteristic of still higher zones. The genus Phyllograptus 

 attains in this zone in Ph. t y p u s its greatest size and 

 variability within the boundary of a species but is also repre- 

 sented by the diminutive Ph. anna, which also extends 

 into the next zone and foreshadows or closes the life history 

 of this peculiar genus. Finally there appears in this zone the 

 first of the coenograptids, which attain their full development 

 not before the middle part of the Champlainic or Lower Siluric. 

 Lapworth 1 reports a similar association, characterized by 

 Didymograptus bifidus, D. extensus, Phyllo- 

 graptus typus, Tetragraptus bryonoides and 

 T. quadribrachiatus from the " Ballantrae rocks " in 

 Ayrshire, south Scotland. Miss Elles lists Didymograptus 

 bifidus as occurring in the middle and upper Skiddaw 



1 Geol. mag. 1889. 3d ser. 6:22. 



