ROCKS OF SCOTLAND. 3825 
Society, on the Silurian Rocks of Ayr and Wigtonshire, which 
added yet further to our knowledge of the fossils of these an- 
cient rocks, and in which, in its published form, the first Scot- 
tish Maclurea was figured and described, though somewhat 
doubtfully, from the imperfect state of keeping of the specimen, 
and under another name. At the meeting of the British Asso- 
ciation held in this city in 1850, Professor Sedgwick read a 
paper on the Geological Structure and Relations of the Fron- 
tier Chain of Scotland, which derived a peculiar value from the 
previous labors of that great geologist in the older Silurian 
rocks of England, and in which he divided our Grauwackes, 
though with much hesitation, especially with respect to both 
the earlier and later beds, into five great divisions, — four of 
them belonging to the Lower, and the fifth probably, as he 
stated, to the Upper Silurian. In comparing the Scottish with 
the Cambrian chains, he remarked that the lowest and oldest 
fossils of both appear to be graptolites ; and in a paper on the 
Graptolites of the Black Slates of Dumfriesshire, by Mr. R. 
Harkness, which appeared in the “ Geological Journal” of 
last year, we find a minute description, accompanied by good 
figures, of these earliest inhabitants of what is now Scotland. 
They are judged to have been zodphites, akin in some of their 
forms to our modern Pennatuladz, and in others, it is supposed, 
to the Sertularia; but the relationship of these last is deemed 
less clear. It is, I suspect, remote in both cases. Some of 
my Girvan specimens of Graptolithus foliaceus,— one of the 
species deemed akin to the Pennatuladz, — exhibit the central 
axis prolonged beyond its double row of cells, but, unlike our 
common sea-pen (Pennatula phosphorea), always at the upper 
end; and in specimens of Graptolithus tenuis, derived from the 
same neighborhood, and which is one of the species regarded 
as akin to the Sertularia, though some of the stems seem fringed 
on both sides with short, oblique, alternate cells, somewhat 
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