a. Zoological Position of the Graptolitide. 57 
_ Barr., but it is very largely developed in a new species of Di- 
plograpsus from the Dumfriesshire Shales, which I have named 
D, vesiculosus (Pl. II. fig. 11). 
The homologies of the solid axis, with its extensions and ap- 
pendages, are by nomeansclear. ‘There is no known structure, 
either amongst the Hydrozoa or Polyzoa, which could be looked 
upon as its exact equivalent; and it is probably related (but by 
analogy only) with the horny or calcareous “ sclerobasis ”’ of the 
Gorgonide and Pennatulide amongst the Actinozoa. Its chief 
function certainly seems to have been to give due support to 
the coenosarc, and to prevent injurious flexion of the pliable 
polypary ; but it probably subserved other purposes of even 
ater importance. No close parallel can be drawn between 
the “‘radicle”’ of the Graptolitide and the foot-stalk of the 
Sertularide, since the former structure beyond doubt did not 
serve as an organ of attachment. The central disk or cup of 
some Dichograpst, Tetragrapsi, and Diplograpsi was compared 
by Prof. Huxley with the basal plate of Defrancia, a Bryo- 
zoon; but I think a more probable homologue is to be found in 
the “float,”’ or “‘pneumatocyst,” of the Physophoride, an 
order of the oceanic Hydrozoa. The distal extension of the axis 
is entirely without a parallel; and when dilated, as in Diplo- 
grapsus vesiculosus, Nich , it is difficult to conceive of any 
function which it can have subserved. It cannot be of the 
nature of a float, since it occupies the distal and not the proxi- 
mal extremity of the organism; and the most probable view 
would perhaps be to consider it in some way connected with 
the reproductive process. The second element, namely the 
“common canal,” is structurally a tube extending along more 
or less of the axis, and giving origin to the cellules. Ordi- 
narily it appears as a flattened space between the cellules and 
the solid axis (Pl. III. fig. 2); and it seems certainly to be a 
distinct structure even in those forms in which the cell-parti- 
_ tions are attached to the axis. As to the intimate nature of 
_ the common canal, there seems to be no doubt that it conveyed 
a soft connecting substance uniting the various polypites into 
an organic whole; and it is therefore homologous with the 
“coenosare” of the Hydrozoa. Its existence consequently 
forms one of the strongest grounds for eliminating the Grap- 
tolitide from the Polyzoa, since no analogous structure is known 
to occur in any of the latter. 
_ Of the cellules there is little to be said. They vary much 
in shape; but they usually constitute more or less cylindrical 
or quadrangular tubes, the bases of which are attached to the — 
common canal, whilst the opposite extremities terminate in 
open mouths—the “ cell-apertures.”” They thus come to re- 
