GRAPTOLITES. . 17 
cellules.* The species of this genus, so far as known, have the cellules 
developed on one side only of the:stipes or branches ; though there seems 
no reason why we should not have species with cellules upon the two 
sides of the axis. 
The species for which I have proposed the name Zhamnograptus con- 
sist of slender cylindrical stipes and branches, some of them very similar 
in general aspect to Rastrites, but the alternating branches are long and 
slender, and we have found no appearance of cellules on any part of the 
specimens known. 
Tn the accompanying illustration the branchlets are given off alternately 
on the opposite sides of a stipe or rachis, and the slender solid axis can 
be traced from the main stipe into and along the centre of the branchlets. 
The analogy between these forms and the celluliferous graptolites of the 
preceding illustrations does not appear to be very intimate, but they 
occur in the same beds, and the fossil has the same texture and substance. 
Fig. 24. 
THAMNOGRAPTUS TYPUS. 
In the genus Ptilograptus (plate xxi) we have a somewhat analogous 
mode of growth, but there is always an aspect or expression of the fossil 
which distinguishes it from Zhamnograptus. In one species of Pizlo- 
graptus moreover, cell-apertures have been detected on one side of the 
branches or pinnulee. 
In the Buthograptus we have a form bearing some analogies with the 
* See illustration of Rastrites Barrandi under section iii, p. 26, of this introduction, 
