54 Transactions. 
Llandovery. It will be observed that it shows no row of denticles 
or hydrothece as in Monograptus sagittarius, Linn, M. Nilssont, 
Barr, and many others, but merely elliptical apertures forming 
the cell’s mouths on both sides of a cylindrical frond or tube, 
which are not opposite to each other, but alternate. Its width 
when fully developed is about one-eighth of an inch. The solid 
axis is prolonged both proximally and distally to a considerable 
length. The external appearance of the frond varies according 
to the amount of pressure it may have sustained. There are 
about 30 hydrothece to an inch. The cylinder or frond of 
Climacograptus teretiusculus (His.) is made up of two semi-circular 
compartments, placed back to back with the septum, and the 
solid axis placed in the centre forming a mutual gable or partition, 
as it were, between the two colonies of Zooids, having each their 
own row of cells and common canal, each colony being quite 
independent of the other. Localities for this species are Dobb’s 
Linn, Hartfell, Beld Craig, &c. 
Genera Didymograptus._The polypery is composed of two 
simple branches springing bilaterally from the radical or initial 
point. The branches are monopriodon, assuming different angles 
in different species. The cells, or ‘“hydrothece,” are generally 
opposite to the radical or initial point, but in some examples they 
are found on the same side. The size of the radical or spine 
varies very much in different species, and in some it is altogether 
awanting. It is not easily determined to what uses these spines 
could have been applied, as there does not seem to be 
any evidence that they had been objects of attachment. If we 
look at their different positions with the spines inside of the 
angles or concave, one would think it scarcely possible for them 
to be used for such a purpose, more especially as some of them 
are without any radical or initial point whatever. The genus 
Didymograptus was originally founded by M‘Coy (1851), and 
takes in those Graptolites which are “bifid,” or of a twin 
character. The vertical range of this genus has its commence- 
ment in the Skiddaw and Quebec groups, and attains its maximum 
in those rocks in which it is represented by numerous and remark- 
able examples. It likewise occurs in the lower Llandeilo, and is 
also well represented in the upper Llandeilo rocks, but is scarcely 
known in the Caradoc period of this country. Several examples 
are got in the rocks of this age in America, but not one is known in 
the upper Silurian, and it may safely be said to be characteristic 
of the upper and lower Silurian periods. 
