108 GEOLOGICAL DISTRIBUTION 
either the primary or secondary divisions ; but there is little doubt 
that the whole of the Tertiary period is only equal to a fraction of 
the Mesozoic as regards its duration, while it is probable that 
the duration of the Mesozoic epoch was largely exceeded by that 
of the Paleozoic. 
Mesozoic Mammals.—The earliest date at which mammals are at 
present known is in the upper part of the Triassic period, which 
forms the base of the great Mesozoic epoch ; and from this date they 
are represented more or less abundantly in various horizons of the 
Jurassic and Cretaceous. 
The very rapid advances in our knowledge of these forms which 
have been made in the last few years, especially in consequence of 
the explorations of rich fossiliferous beds in North America, have 
not only completely changed the present aspect of the science, but 
give such promise for the future, that any sketch which we may 
now attempt of this branch of the subject can only be regarded 
as representing a transient phase of knowledge. It will be well, 
however, to gather together in this place the leading facts now 
ascertained with regard to the most ancient forms, as, owing to the 
uncertainty of their relationship with any of the existing orders, 
they will be most conveniently treated of separately, while the 
ascertained facts relating to the geological history of the forms 
more nearly allied to those now living will be more appropriately 
described under the account of the different groups into which the 
class may now be divided. 
The remains of mammals which existed anterior to the Tertiary 
period hitherto discovered nearly all belong to creatures of very 
small size, many of the largest scarcely exceeding the common Pole- 
cat or Squirrel. Some are known only by a few isolated teeth, 
others by nearly complete sets of these organs, and the majority by 
more or less nearly perfect specimens of the rami of the lower jaw. 
It is a very curious circumstance that this part of the skeleton 
alone has been preserved in such a large number of instances. 
Only very rarely has a nearly complete cranium been found; and 
there is no satisfactory evidence of the structure of the vertebral 
column of any single individual, and only one known case of a com- 
plete limb.!' The species already described from European strata 
are numerous, although the number of genera and species has lately 
been reduced. Of these by far the greater number have been found 
at a single spot near Swanage in Dorsetshire, in a bed of calcareous 
mud only forty feet long, ten feet wide, and averaging five inches in 
depth. The marvellous results obtained by the exploration by Mr. 
S. H. Beckles of this small fragment of the earth’s surface show by 
what accidents, as it were, our knowledge of the past history of life 
1 The fore limb from S. Africa described as Theriodesmus, which appears to 
be mammalian, and may belong to T'ritylodon. 
