BOTANY OP TERRA AUSTRALIS. 61 



exists only on this inner branch of the vein, as somewhat 

 analogons to the double involuerum of Diplazium ; but in 

 another point of view it may be regarded as an approach 

 to the structure of Nephrodium, to which this plant has 

 been improperly referred. 



There are some other Australian natural families of [sse 

 plants to whichj either as containing distinct and peculiar 

 genera, or a considerable number of species, similar remarks 

 might be extended ; but I have already exceeded the hmits 

 prescribed for the present essay, which I shall therefore 

 conclude with a few general observations, chiefly deduced 

 from the facts previously stated, and with a very slight 

 comparison of the vegetation of Terra Australis with that 

 of other countries. 



I have formerly remarked that nearly half the Australian 

 species of plants, at present knoAvn, have been collected in 

 a parallel included between 33° and 35° S. latitude ; and it 

 appears, from the preceding observations on the several 

 natural orders, that a much greater proportion of the pecu- 

 liarities of the Australian Flora exist in this, which I have 

 therefore called the principal parallel ; and that many of 

 them are even nearly confined to it. But these peculiarities 

 exist chiefly at its western and eastern extremities, and 

 are remarkably diminished in that intermediate part which 

 is comprehended between 133° and 138° E. long. 



From the principal parallel most of the characteristic 

 tribes diminish in number of species as well as of individuals, 

 not, however, equally in both directions, but in a much 

 greater degree towards the equator. In Van Diemen's 

 Island the same general aspect of vegetation is retained; 

 but of the natural orders forming the peculiar character of 

 the principal parallel several are very much reduced, while 

 none are augmented in numbers ; and the only tribes which 

 enter in nearly the same proportion into the composition of 



