56 Dr. H. A. Nicholson on the Nature and 
The “solid axis’ is one of the primitive elements in the 
formation of every Graptolite. In the simple os 
species it seems to be a solid cylindrical rod (Pl. ILI. fig. 2); 
but in the biserrate forms it is certainly a corneous plate, di- 
viding the frond into two vertical compartments (PI. III. fig. 3), 
apparently composed of two laminz, with a median cylindrical 
rod and perhaps including a central canal. The axis may be 
prolonged beyond one or both extremities of the celluliferous 
stipe; and for convenience I shall term these Pax spr! the 
basal or “‘ proximal ”’ and the terminal or “ distal” extensions 
of the axis. These prolongations (Pl. ILI. fig. 4) are little 
understood; but they require especial attention, as throwing 
great light upon the true nature of the Graptolitide. The proxi- 
mal extension of the axis is present in probably all the true 
Graptolites, and constitutes the ‘ radicle” or “ initial point” 
of Hali. The radicle varies greatly in length, and it may 
consist of the solid axis alone, as in Diplograpsus teretiusculus 
&c.; or it may be bordered by the common canal prolonged 
upwards into the first cellule or cellules, as in G. sagittarvus, 
Linn., Diplograpsus cometa, Gein., and D. acuminatus, Nich. 
Again, in most of the branching and complex Graptolites 
the solid axes of the various simple stipes composing the frond, 
together “pti with the common canal, are prolonged ea 
mally and are united into a connecting process, which is always 
destitute of cellules and is termed the “ funicle” by Hall, as is 
seen in the genera Dichograpsus and Tetragrapsus (P\. II. 
figs. 5, 6, 15, 16, and 20), | 
Lastly, the subdivisions of the funicle may be embraced by 
a central corneous disk or cup, which is apparently composed 
of two lamin, and probably enclosed a central cavity. This 
corneous cup. (fig. 6) is’ best seen in several species of Dicho- 
grapsus and Tetragrapsus ; but it also exists in some specimens 
of Diplograpsus bicornis, Hall (figs. 8, 9, 10), and in a new 
species of Diplograpsus from the Upper Llandeilo rocks of 
Dumfriesshire, which I have named D. physophora (fig. 7). 
The distal extension of the solid axis is only seen in the Di- 
plograpst, and possibly in Retiograpsus, Hall, and Retiolites, 
Barr., seeming to be merely accidental when seen in the adult 
monoprionidian forms, as it rarely is. This distal extension 
of the axis usually consists of the solid axis alone, as in all 
the common Diplograpsi; but it may consist of a bladder-like 
body, more or less elliptical in form, with a distinct filiform 
margin and of uncertain function. This vesicular dilatation 
seems always to be a direct expansion of the axis, which would 
thus appear to be tubular. It is seen to a very moderate ex- 
tent in some specimens of D, pristis, His., and D. palmeus, 
