78 Mr J. E. Marr, On Homotavis. [Feb. 14, 
Again, it is very noteworthy that the change in the character 
of the faunas is accompanied by change in what may be termed 
accidental characters of the rocks which contain those faunas,— 
accidental inasmuch as it is difficult to see how these characters 
could produce any effect upon the organisms. Taking the same 
three faunas, we find that the shales containing the Birkhill faunas, 
are usually marked by a comparative paucity of light green bands, 
whereas the Gala beds contain an abundance of such, as seen not 
only in Britain, but also in Sweden and Bohemia. As the grap- 
tolites are generally absent from these green bands, and occur 
in the dark shales interstratified with them, it is hard to under- 
stand how their relative abundance can produce any effect upon 
the faunas. Still more marked is the occurrence of large elliptical 
concretionary nodules in the beds containing the Riccarton fauna, 
in Britain, Scandinavia, France, and Bohemia. These nodules are 
never, so far as I am aware, discovered in the rocks containing the 
Birkhill and Gala forms of life. 
So much for the facts presented by an examination of the 
graptolitic deposits. Before discussing these, I may call attention 
to two cases of non-graptolitic rocks. In the Lake district, a 
band of rock, the Coniston Limestone, is succeeded by a thin lime- 
stone, which I have spoken of as the Staurocephalus zone, and 
this in turn by black shales. This Staurocephalus limestone is 
only a foot or two in thickness, nevertheless a similar fauna to 
that which it contains, is found in a similar position in Wales, 
Scotland, and Scandinavia, and probably elsewhere. The litho- 
logical characters of this band, which are peculiar, are also re- 
markably constant. It is noticeable also, that although in the 
Lake district it reposes upon a limestone, and is succeeded by 
shales, its fauna is entirely different from that of the underlying 
limestone, and closely similar to though richer than that of the 
succeeding shales. This seems to indicate that the change in 
the fauna was not produced by a change in the supply of 
sediment. 
The close similarity of the Ammonite zones of the Jurassic rocks 
in Britain and Germany is too well known to need illustration. 
C. Difficulties in the way of abandoning the wew of con- 
temporaneity. These are two-fold. Firstly, those connected with 
the migration of the organisms. Jt has been seen that certain 
dominant genera appear in like order over wide areas, and that 
speaking generally they disappear in like order also. This is only 
conceivable upon the supposition that they originated in the same 
region, and that in each case the direction of migration was the 
same, which is very unlikely, or that the time taken for dispersal 
was short, as compared with the time during which each genus 
