BRACHIOPODA—THOMSON. 53 
DISCUSSION OF THE DISTRIBUTION. 
There are gaps of different kinds in our knowledge that preclude any final con- 
clusions as to the manner in which the present distribution of southern brachiopods 
has been effected. The first of these is our ignorance of the fauna of large strips of the 
coastal seas. Schuchert considers that brachiopods have been so assiduously collected 
that little will be added to our knowledge of the distribution of the species. Since there 
are but few cases of discontinuous distribution in the southern seas, It is not probable 
that the range of the known species will be greatly extended, except along the coasts 
from which they are already known, but there is every reason to suppose that a very 
large number of new species will yet be found between the 100 and the 1,000 fathom 
line, where the dredgings are few. Fischer and Oehlert gave in 1892 a list of five species, 
belonging to four genera, from Bass Strait and the coast of Tasmania. From this part 
of the Australian coasts twelve species belonging to eleven genera are now known. 
Again, within the last three years the known fauna of the New Zealand coast has been 
more than doubled. There can be little doubt that further dredging would bring to light 
many new forms, especially on the West Australian, Madagascar, South African, and 
South American coasts. Even for the very shallow water genera, such as Megerlina, 
it is unsafe to presume that the distribution is accurately known, as there have been 
no systematic students of the class in Western Australia and in South Africa. 
The second gap in our knowledge lies in the incompleteness of our recognition 
of distinct genetic stocks. Some students object to the process of minute distinctions 
between genera, mainly on the ground that it renders the study more complex, and @ 
matter only for the specialist. But the narrow definition of genera, if it is based on 
phylogenetic grounds, prevents the assimilation of apparently similar, but historically 
distinct forms, and for the purpose of discussing geographical distribution and geological 
correlation becomes an instrument of the utmost utility. It is only necessary to specity 
the genus Magellania sensu lato, which has been fully discussed above, in illustration 
of this point. When all the Recent and Tertiary species of the southern hemisphere 
have been correctly placed genetically, it may be possible by the aid of brachiopods 
alone to gain a fairly accurate idea of the latest former land connections of the Southern 
Hemisphere. 
There is a gap in our knowledge of still another kind which can hardly be remedied, 
and must be made the best of. That is caused by the absence of Tertiary marine rocks 
in South Africa and the western coast of Australia, and in that part of the Antarctic 
Continent facing South Africa and Australia. This absence of Tertiary rocks can only 
mean that these lands existed as such in Tertiary times, and then extended further 
seawards, since apart from probable faulting marine erosion alone must have since 
encroached some distance landwards. The marine beds marginal to the Tertiary lands 
have never become raised above sea level, else remnants of them would surely have 
been preserved in the present continents and islands by unequal uplift or subsequent 
down-faulting. The Tertiary faunas of these coasts can never become known until 
it is found practicable and desirable to obtain borings from the sea-bottom, 
