ROCKS OF SCOTLAND. 333 
enough to procure a complete Chetrurus gelasinosus,—a spe- 
cies of which Sir Roderick figures from this deposit a detached 
head, and which, though I find no trace of it in his “Silurian 
System,” is described by M’Coy as a not rare trilobite in the 
Sdurian rocks of Kildare. It is a circumstance not unworthy 
of notice, that the Scottish Silurian fossils are more completely 
identical with those of the Irish than with those of the Eng- 
lish group. They are Celts, if 1 may so speak, rather of the 
old Scoto-Irish than of the old Welsh type. I have said that 
to the south of Girvan these trilobite shales are rich in grap- 
tolites and orthoceratites. ‘The graptolites are usually of that 
double fringed section (diprion), with an axis in the centre, 
to which the typical Graptolithus foliaceus belongs; the only 
exceptional species, so far as I know, being Graptolithus tenuis, 
a member of that single fringed section (monoprion) repre- 
sented by the typical Graptolithus Ludenses. Associated with 
these, but rarely, we sometimes find a large dark-colored lin- 
gula, probably the Lingula ovata of M’Coy, also a Kildare 
species; and Orbicula crassa, a finely stratied shell, bearing 
usually the same dark hue, as if both organisms had been coy- 
ered by an epidermis, which had alone survived when their 
shelly substance had been absorbed in the rock. The orthoce- 
ratites of the deposit exist in a peculiar state of keeping. They 
have been converted, with the filling of all their chambers, into 
a pure chocolate-colored lime ; whilst the gray shale in which 
they lie is so little calcareous as to remain impassive under 
the strongest acids. Many of them seem to have been broken 
across ere they were committed to the rock, and exist as de- 
tached though very entire fragments, consisting of from six to 
ten chambers a-piece. It is stated by Sir Roderick Murchison, 
that one of the largest of the Girvan orthoceratites being of a 
kind unknown to him, he referred it to M. Barrande, then on 
a visit to our conntry, who recognized it as a Bohemian spe- 
