exists only on this inner branch of the vein, as somewhat analogous to the double involucrum 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 [586 plants to which, either as containing distinct and peculiar genera, or a considerable number of species, similar remarks might be extended; but I have already exceeded the limits 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 known, 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 peculiarities 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
