GRAPTOLITES. 47 
The Canadian specimens which I have referred to this genus are so 
extremely attenuated that it is impossible to determine the details of 
structure, and the surface-characters are obscure. ‘Thus far we have 
no American specimens in a condition to afford the means of elucidating 
some obscurities which seem to me still to exist in regard to the intimate 
structure of this genus. The species of the Clinton formation is extremely 
compressed ; and while some specimens show the cell-divisions termmating 
at a distance from the centre, yet, after protracted and repeated exami- 
nations, my most critical observations result in showing only the struc- 
ture which is illustrated on plate B, figs. 20 and 21. 
On one side we have an external, cylindrical, solid axis, to which the 
cell-divisions are joied; but these latter show only filiform cylindroid - 
processes, extending from the axis to the cell-margin, and projecting a 
little beyond the margin of the stipe. The only other aspect which we 
observe in this species is that of an undulating or zigzag filiform axis on 
the opposite side, to which the cell-partitions are joined, as in fig. 21, 
plate B. We know this to be on the opposite side or within the stipe, as 
it is sometimes seen overlying the straight axis and cell-partitions. 
_ At the junction of the cell-divisions with the zigzag axis there are other 
processes of similar character, projecting upward and outward from the 
‘axis, all of nearly equal length, but apparently broken at their extremi- 
ties. J have not been able to determine any connection between these 
and other parts of the skeleton, but we have the two structures very 
clearly represented in the figures referred to. I have supposed that 
similar processes may have extended to the opposite side, from the junc- 
tion of the cell-partitions with this undulating axis, either joining the 
cell-divisions or the straight axis; but after long mvestigation I have been 
unable to find satisfactory evidence of such connection. The cell-apertures 
are surrounded by thickened margins, and the only appearances of cell- 
partitions are the sub-external cylindrical extensions from the aperture to 
the axis. Neither the species of the Quebec group nor that of the Clinton 
formation, in any of the specimens seen, are in a condition to show 
evidence of the concavo-convex character of the stipe represented by 
M. Barrande and Prof. Geinitz. 
The species of Retograptus, while having some characters in common 
with Ketiolites, do not possess the reticulated structure of the test in either 
of the described species. There yet remains some obscurity in regard 
to the internal structure of this genus, which can only be satisfactorily 
explained by the examination of better-preserved specimens. Neverthe- 
less, in its general form, structure, and mode of growth, it is shown to be 
quite distinct from other graptolitic genera. The three species referred 
to this genus present differences which can only be reconciled by supposing 
that the two sides of the stipe are very unlike each other in form and 
external characters, as in [etiolites. - 
