DESCRIPTIVE REMARKS. XXiil 
Their exact relations are still, however, the subject of discussion, not- 
withstanding the discovery of so many extraordinary forms in Canada; 
some naturalists believing their alliance to be more with the Polyzoa. 
The forms represented on Plate ix. are examples of the various 
genera met with in British strata; in Graptolithus, a single row of 
cellules are arranged on the axis or stipe, of which G. sagittarius, 
fig. 1, may be considered the type; G. Nilssoni, fig. 2, being another 
example of these single Graptolites, which are not confined to Lower 
Silurian strata, like the double forms; one of the single Graptolites, 
G. priodon, being almost as abundant and generally distributed as D. 
pristis, ranging from Caradoc or Bala rocks through all the stratigra- 
phical divisions of the Upper Silurian. Those haying a central axis, 
on which a double line of cellules are arranged, are well shown in 
Diplograpsus pristis, and its variety scalariformis, fig. 3. To this species 
the living Zoophyte, Virgularia mirabilis (woodcut, fig. 4, a, b), pre- 
sents perhaps the nearest resemblance : it is the most frequent of all the 
Graptolites in the Llandeilo Formation, of which it is eminently charac- 
teristic, and, as far as at present known, confined to Lower Silurian 
strata, as are all the double forms. 
D. teretiusculus, fig. 4, also a double form, is a smaller and less 
abundant Graptolite, although a characteristic one of the Llandeilo 
series, to which it is said to be entirely confined. 
In the double Graptolite, Didymograpsus Murchisoni, fig. 5, the 
axis diverges from a ‘‘radicle,” or point, the cell serratures being ar- 
ranged in a single’series on the inside of each branch. Another and 
smaller diverging form of Graptolite is that of Didymograpsus sextans, 
fig. 6; it has the cell serratures arranged on the outer edge of the axis, 
and is very characteristic of this Formation. Some of the specimens col- 
lected by Mr. C. Galvan, of the Geological Survey of Ireland, from near 
Dungarvan, show the ‘initial points,’”’ or ‘ radicles,”’ exactly resem- 
bing the original figures of this species given by Professor James Hall, 
in the Paleontology of New York, vol. i., pl. lxxiv., fig. 3, d, e.* 
As an example of a coiled form, Rastrites peregrinus, fig. 7, is not 
an uncommon one in the black shales of the upper division of this For- 
mation in Dumfriesshire. This elegant little Graptolite differs from all 
the preceding ones alluded to in the narrow and delicately curved axis, 
from which proceed, at regular intervals, the elongated narrow tubular 
cellules peculiar to this species, and developed on one side only of the 
axis or stipe. 
In concluding this account of the Fossils of the Llandeilo Formation, 
it will be important to remember, that Graptolites, according to our 
present knowledge, wherever detected, will be found to present a cer- 
tain indication of Silurian strata. This fact, in connexion with the 
merest rudiments of geology, if properly known, might have saved 
* Those who wish to study this interesting and important group of Fossils should 
consult Decade ii. of the Geological Survey of Canada, under the direction of Sir Wm. 
Logan, the Graptolites, by Professor James Hall, 1865. 
