58 Dr. H. A. Nicholson on the Nature and 
semble somewhat the “ hydrothece” of the Sertularide ; but 
they are invariably in contact for a greater or less portion of 
their length, and they do not seem ever to be cut off from the 
common canal by any partition or diaphragm. 
Development.—The ordinary germs or embryonie forms 
of Graptolites (Pl. III. figs. 12, 13), in the youngest condition 
in which they are preserved to us, are minute corneous bodies, 
consisting of a small radicle or mucro, which is in fact the 
commencement of the solid axis. ‘This, the primitive structure 
of the embryo, is widened out distally by the presence of the 
common canal on one or both sides, according as the germ 
belongs to a mono- or to a diprionidian species ; and finally 
one or two primordial cellules are superadded. Even at this 
stage the solid axis projects beyond the primitive cellules as a 
naked rod ; and its growth was probably carried on to a certain 
extent independently of the rest of the organism. These germs 
are various In size, and differ in minor details ; but they ail pos- 
sessed a corneous envelope, and they cannot safely be compared 
to any of the embryonic forms of the existing yaw 
or Bryozoa. It must, in fact, be considered very probable 
that these germs, as we see them, are considerably advanced in 
growth, and that the earliest form of the embryo was devoid of 
any corneous test. 
As to the further development of the stipe, it must suffice to 
state that in the simpler genera the secondary cellules appear 
to be intercalated between the initial point or radicle and the 
primordial cellule or cellules, so that the youneass cellules are 
proximal, the oldest distal in position. This mode of develop- 
ment corresponds with that observed in the Calycophoride and 
Ph ih ke amongst the Hydrozoa. 
Fae Dati quite of late years, the reproductive 
process in the Graptolitide was quite unknown ; and even now 
our knowledge is partial and to a great extent inferential. In 
1858 Prof. Hall gave a description of some specimens of D2- 
plograpsus Whitfeldii bearing bodies which he regarded as 
ovarian vesicles. According to Hall, these appear as small 
ovate buds attached to the stipe on both sides, enlarging to 
form elongated sacs, and finally dehiscing. ‘These sacs are 
limited along their margins by a filiform border like the solid 
axis of a Graptolite ; and it appears inconceivable that their 
function can have been other than reproductive. , 
At the meeting of the British Association in 1866, I 
drew attention to the occurrence in the Graptolitic rocks of 
Dumfriesshire of bodies essentially similar to those described 
by Hall; and I have since described them more fully and 
_have adduced instances in which they are seen attached to 
