42 CANADIAN FOSSILS. 
or value in the application of these terms: we are thus reduced to the 
necessity of adopting the old term Graptolithus. Again, the value of 
Didymograptus I conceive to be pretty well illustrated in the case of 
G. caduceus, the original of which is cited from Lauzon, Canada.* After 
studying the large collections of graptolites made by the Canadian Geo- 
logical Survey, I am compelled to believe that the G. caduceus was 
founded upon such forms as I have represented on plate xvi, figs. 22, 23, 
and 24 ; for we have no two-stiped species or forms of “‘ Didymograptus”’ 
with a pedicle or radicle so long as that represented in the figures of 
Mr. Salter, nor any one so abruptly recurved; and I regard the apparent 
radicle in the two examples figured as simply one of the four stipes 
imbedded in the shale, and exhibiting its non-celluliferous margin and a 
small portion of its width, as I have shown in the figures cited. 
Other varieties of this form show only the two simple stipes, with a 
slight process in the centre. We have therefore a “‘ Zetragraptus” na 
condition undistinguishable from a ‘* Didymograptus’’; and the same may 
happen in G. bryonoides, as shown in plate iv, fig. 5, where the quadri- 
partite stipe is separated into two; and in the separated stipes it is mpos- 
sible to know if there have been two, four, or eight in the entire indi- 
‘idual. With regard to those fronds which are repeatedly dichotomous, 
forming the genus Dichograptus, of which the distinguishing character is 
the central ‘‘ corneous plate which envelopes all the lower part of the 
branches,’ we may remark, that we have three or four species of the 
four-stiped form, or “* Zetragraptus” with the central corneous plate ; while 
we have four species which are not known to possess it. Of the two 
eight-stiped species known, one has the central corneous plate or dise, 
and the other was probably destitute of such an appendage. In 
G. Logani, with its numerous simple stipes, the central corneous plate 
is usually present, though not in all examples; while G. multefasciatus, 
with more numerous simple stipes than G. Logani, is not known to 
have a central corneous disc, and, from its mode of growth, probably 
never possessed such an appendage. From the irregularity of growth in 
the G. abnormis, I infer that there was no central plate. 
In all the properly-branching species where the initial point is known, 
as in G. flexilis, G. rigidus, and G. Milesi, no such central plate has 
ever been seen; nor has it been shown in any European species, so far as 
I know. The frequently-bifurcating stipes, similar to the one originally 
proposed by Mr. Salter}t as the type of Dichograptus, are not known to 
possess the central corneous disc. 
Although entirely willing to accept and adopt such subdivisions of the 
graptolites as will aid in determining their zoological character and 
‘ 
* Graptolithus caduceus, Salter; Quarterly Journal of the Geol. Society, vol. ix. 
} Geologist, vol. iv, p. 74, 1861. 
