io8 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 Palaeozoic. 



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. 1 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 Tritylodon. 



