8 
sufficient for the purposes of identification ; but, on the other 
hand, it is known that the appearance of the same stratum 
frequently varies so greatly, even at distances not very 
remote, that portions from diff'erent localities bear but little 
resemblance to each other, and have been supposed to belong 
to different beds. Whole geological formations have been 
thus supposed to belong to diff'erent geological epochs, when 
the general appearance produced by the physical character 
of the rocks was solely relied upon, whereas such rocks, 
remotely separated, and of very different physical character, 
when investigated by the light derived from the examination 
of their organic contents, have been proved to be eo-tem- 
poraneous deposits. On the smaller scale, even, individual 
beds are sometimes capable of being thus identified by their 
accompanying fossils, when their physical characters alone 
would fail to determine their relative position. A remark- 
able illustration of this fact occurs in our own district. At 
Moortown, three miles north of Leeds, is a bed of a black 
earthy looking substance, about four inches in thickness ; 
and this is identical with the beds of coal, several feet in 
thickness, which are worked at Halifax, and supply the 
extensive manufacturing districts of Bradford and Halifax 
with the greater part of their fuel. But, how do we know 
the thin bed of black earth at Moortown to be the same 
bed as the coal at Halifax? Their appearance, or in other words, 
their physical characters, do not enable us thus to decide. 
It is from their being accompanied by a particular fossil, the 
Pecten Papyraceus, which is now known to be characteristic 
of this bed. The shale which forms the roof of the Halifax 
coal is crowded with this beautiful fossil, and by means of this 
shell, may be traced, overlying the black earth of Moor- 
town, the impure coal of Kirkstall wood, and the progres- 
sively improving coal of Idle, Bradford, and Halifax. All 
fossils, however, are not of equal value for the purposes of 
identification. The Pecten Papyraceus is of great import- 
ance in this respect, for, although occurring extensively in the 
mountain limestone formation, it is only known to prevail in 
the Yorkshire coal measures in the single bed of shale which 
forms the roof of the Halifax coal. On the contrary, other 
fossils, as the Unio Acutus and Unio Constrictus, may have 
so wide a distribution as to be of but little diagnostic 
value. Before, then, we can determine the diagnostic value 
of any fossil species, genus, family, or class of the animal 
kingdom, a careful and extensive investigation is required as 
to the precise range of its distribution. Throughout the 
whole animal kingdom there is no class more calculated, 
by the examination of its fossil remains, to aid the 
geologist than fishes. This class, placed in the verte- 
brated division of the animal kingdom, in that divi- 
sion which includes even man himself, is thereby associated 
