IIIXD : CARI50XIFER0US KOCKS OF THE PEXXINE SYSTEM. 457 
natural state of things for a muddy environment would be extremely 
distasteful, if not fatal, to organisms which lived in a clear sea. I have 
never yet obtained any of the Nuculidce in pure limestones, but the 
species of these genera always are found in muddy deposits. Of 
brachiopods, the Lingulce, Discina nitida, Orthis Michelini, Chonetes 
Laguessiana, Rhynchonella trilatera, Athyris amhigua were those 
most able to live in muddy waters, and are not commonly found 
in pure limestones. 
Prodiictus giganfeus, P. lafissimus, Chonetes papilionacea, 
and Amplexus coralloidcs, and the great majority of the corals and 
Polyzoa are characteristic of the whole Carboniferous Limestone 
series, in which mass of rocks I have no evidence of life zones at present. 
For, although fossils are more numerous at the top, I cannot find 
that any species are confined to any definite horizon. It is probable 
that the rarity of fossils in the middle and lower beds of the thick 
massif limestone of the North Midlands is due to the fact 
that metasomatic changes have obliterated the fossils, but it is 
probable that a microscopic survey of the different beds of the massif 
might reveal some microscopic forms which had a limited vertical 
distribution. When one considers the repeated changes in the 
conditions of the sedimentary deposits which form the Yoredale 
phase of the Carboniferous rocks, it is a remarkable fact that the 
various limestones and marine shales are not characterised by definite 
fossils, but that the faunas of similar deposits at several horizons are 
identical. This condition of things is doubtless due to the very 
local character of the deposit of shales, and that the limestone fauna 
had not far to migrate southward when its present habitat was ren- 
dered unsuitable for it by the incursion of muddy water, and con- 
sequently had to travel back a very short distance to that area when 
muddy conditions ceased. In other words the Carboniferous Lime- 
stone fauna flourished, as a whole, right through the deposit of the 
massive beds, and advanced north or retreated south whenever 
clear conditions obtained in the area occupied by the Yoredale phase 
of rocks. Consequently the various limestones of the Yoredale 
Series contain the same faunas, and similarly the intercalated marine 
muds also contain the same faunas. 
