220 MR. J. W. KIRKBY ON FOSSILS IN THE 
almost invariably present wherever the Lower and Middle Lime- 
stones are fossiliferous, are mostly absent; and the one or two 
which are present are extremely rare. 
Whatever may have been the originating cause of this differ- 
ence in the distribution of the Lower Limestone fossils, it is, I 
think, evident that there existed even in the Permian period 
distinctions in the geographical distribution of species, analogous 
to those we observe in the faune of recent seas. Nor is this the 
only instance that occurs among Permian fossils, that might be 
quoted against the still prevalent notion that the distribution of 
palzozoic marine life was essentially different, in the wider or 
almost universal range of species, from that of existing marine 
life. For though the Permian species of Britain certainly form 
one general fauna, they yet occur under such different arrange- 
ments in the various districts where Permian rocks are found, as 
to show that the old paleeozoic sea, wherein these rocks originated, 
was not peopled throughout its whole area by precisely the same 
arrangement or grouping of species. For instance, while the 
floor of the Lower Limestone sea in Durham was covered with one 
or more groups of species, the floor of the same sea in South York- 
shire was peopled by a different group, or at least by one so dis- 
similarly constituted as not to have one prevailing species in com- 
mon. Thisis not the place for me to enter further into the details of 
the question, else I could easily have mentioned other examples. 
I will only add that I am not disposed to look upon the fauna 
or faune of the Permian period as peculiar in this respect, for I 
believe the other paleeozoic fauns have only to be studied with a 
little philosophy in order to bring to light similar differences and 
peculiarities in the distribution of species. 
XVII.—On the occurrence of Fossils in the highest beds of the 
Durham Coal Measures. By James W. Kirxsy. 
Some two or three years ago Mr. Vint of Sunderland showed me, 
from his collection, a small piece of ironstone the surface of 
which was crowded with impressions of a rather minute, ovate or 
nearly circular shell. This specimen had been collected by him 
