GRAPTOLITES. 51 
Il. 
Species having a common trunk or stem, or growing in sessile groups of stipes from 
a common origin, without distinct bilateral arrangement of the parts. Cellules in single 
series on one side of the stipes or branches, and arranged along a common canal or axis. 
1. Branches free (i. e. not connected by transverse bars;) 
cellules in contact or closely arranged. } Denprograptus, Hall. 
bo 
. Branches unfrequently and irregularly connected by 
CALLOGR ph 
transverse processes. } craptus, Halé 
OO 
. Stipes and branches more or less regularly united in a 
reticulate frond, without elongate stem. | Dictyonema, Hall. 
4, Stipes round or flattened, growing in groups, and bifur- 
cating above; margins denticulate; surface rough or 
: Inocautis, Hall. 
scaly. [The relations of this genus are not fully deter- om , 
mined. | 
III. 
Slender cylindrical branches, with tubular cellules arranged in single (or in double 1) 
series. Cellules not in contact in any part of their length. Rastrites, Barrande. 
Iv. 
Species having a common axis or rachis, with slender lateral alternating branchlets, 
Cellules unknown. THAMNOGRAPTUS, Hall, 
Vie 
Species having a common axis, more or less frequently bifurcating, with pinnule 
closely and alternately arranged on the opposite sides; cell-apertures on one face of 
the pinnule. Prinograptus, Hall. 
VI. 
A simple flexuous rachis, with slender flexuous flattened pinnule arranged in alter- 
nating order at close and regular intervals on the two sides. Cell-apertures unknown, 
or circular. Burnoeraptus, Hall. , 
VII. 
Strong stems, which are numerously branched. Branches and branchlets slender, 
arranged in whorls. Cellules undetermined. OupHamia [7], Forbes. 
§ VII.— GEOLOGICAL AND GEOGRAPHICAL DISTRIBUTION OF THE GRAPTO- 
LITES IN THE ROCKS OF CANADA AND THE UNITED STATES. 
Until the remarkable discovery of the graptolites of Point Lévis in 
1854, the chief repository of these fossils known in American rocks was 
in the shales of the Hudson River valley. 
The position assigned to the rocks of the-udson River valley was the 
superior part of the lower division of the Silurian system. In this respect, 
the horizon of the Graptolite beds corresponded with those of Ireland, 
from which these fossils had been described by General Portlock ;* and 
with the position assigned to those in Sweden, as well as with those of the 
* Geological Report on Londonderry, etc., page 317-322. 
