THE GEOLOGICAL RECORD 205 



be in universal use among the geologists of the world. The 

 exact limits of each of these great divisions have been more ac- 

 curately determined, but the abrupt change in the life-forms, 

 and the world-wide unconformity in the stratification on pass- 

 ing from one division to the other, are as great as ever. Tlie 

 Primary or Palaeozoic period is still that of fishes and Am- 

 phibia; the Secondary or Mesozoic, that of reptiles, in 

 amazing abundance and variety; and the Tertiary or Caino- 

 zoic, that of an almost equal abundance of Mammalia, and 

 with a considerable variety of insects and birds. 



The exceptions to the generality of this classification are 

 few, and are particularly interesting. Of the myriads of rep- 

 tiles that characterise the Secondary era, only two of the nine 

 orders into which they are subdivided have been found so far 

 back as the Permian, the latest of the Palaeozoic formations. 

 One of these most primitive reptiles has a near ally in the 

 strange, lizard-like Hatteria still surviving in some small 

 islands on the coast of l^ew Zealand ; while others which seem 

 to form connecting links with the earliest mammals may be 

 the ancestral form from which have descended the unique 

 types of the lowest living Mammalia, the omithorhynchus and 

 echidna of Australia. 



So with the highest type of vertebrates, the mammals. 

 About the middle of the nineteenth century small mammalian 

 jaws with teeth were discovered in w^hat was known as the 

 dirt-bed of the Purbeck (Jurassic) formation at Swanage; 

 others in the Stonesfield Slate of the same fomiation; and at 

 a later period very similar remains were found in beds of the 

 same age (and also in the Cretaceous) in Xorth America. 

 These are supposed to be primitive insect-eating Marsupials 

 or Insectivora, and were all about the size of a mole or a rat; 

 and it is a striking example of the imperfection of the geo- 

 logical record, that although they occur through the whole 

 range of the Secondary period, from the Trias to the Cre- 

 taceous, their remains are still exceedingly scanty, and 

 they appear to have made hardly any structural progress in 



