BY R. M. JOHNSTON, F.L.S. 141 



the close of the period a great part of the species are 

 found to be identical with existing forms. The great 

 dominant reptiles and batrachians, which gave such a 

 singular character to the Secondary period by their num- 

 bers and variety, have mostly disappeared from the scene, 

 and their places are occupied by the placental and aplacental 

 mammahan types, most of which prevail to the present 

 day. The placental forms, however, are almost entirely 

 unrepresented in the Australasian region, but there instead 

 the aplacental or pouched animals of the kangaroo and 

 wombat type have attained their highest state of develop- 

 ment in size, number, and differentiation. Such being the 

 case, it is clear that, whatever agreement there may be 

 found to exist within the Tertiary period, the subdivisions 

 of the epoch in opposite hemispheres cannot offer a very 

 close correspondence with each other, and the associated 

 forms of life typical of a given formation in one hemisphere 

 would be of little assistance in approximating the boun- 

 daries or relationships of any of the subdivisions of the 

 other hemisphere. 



Classification. 



The only general standard for determining the respective 

 subdivisions of the epoch is the local order of succession of 

 distinct formations, aided, as regards the life of the period, 

 by relationship with existing types as indicated by the per- 

 centage of forms which are in common. 



This latter method has been adopted with success by 

 European geologists, so far as it is applied locally. For 

 purposes of classification the shell-bearing molluscs are 

 generally selected as the most useful and convenient elass 

 of organisms, because they are so abundant and so per- 

 fectly preserved in all countries, both in land, freshwater, 

 and marine deposits ; and are, moreover, from their 

 diversity of specific form, with numerous varieties produced 

 by change of habitat, so useful in indicating the changing 

 conditions of their environments. 



It is true the persistency of certain forms and the varia- 

 bility of others cause perplexity at times; but, upon the 

 whole, the evidences of the conditions under which they 

 lived and of their succession offer greater facilities for the 

 proper classification of rocks than are presented by any 

 other class of organisms. 



