114 SUNKLANDS OF PORT PHILLIP BAY AND BASS STRAIT 
existence of ordinary humus. The distribution of the freshwater 
mussel, the River Blackfish, and the platypus indicates rivers 
flowing from the mainland and Tasmania, joining a trunk stream 
having an outlet into the ocean. The presence of freshwater 
pools, swamps, and billabongs is implied by the migration of the 
frogs, etc. 
This is the ecology of a long-established flora and fauna. The 
open forest was probably on the interfluves of the Tamar Major 
River and its tributaries. The denser forest, it is assumed, was 
on the river flats and the estuary, and was comparable to that in 
the Otway Peninsula and Oippsland in Victoria, and to that of 
northern Tasmania behind the psammaphilous vegetation of the 
shoi-e line. There is, however, evidence that the whole surface 
of the interfluves was not covered with soil: in the National 
Museum of Victoria collection at Melbourne is a piece of polyzoal 
limestone, weighing about 2 cwt., dredged by the Endeavour many 
miles south of the mainland. 
During the post-Yolandean subsidence of the land surface the 
eastern and \vestern ridges became, during the interglacial stages, 
the Bassian Isthmus and the King Island Isthmus, and, with 
continued subsidence, the Bass Strait islands; the King Island 
Isthmus was the first to he breached. These islands mark the 
higher points of the land bridges, and much of them has never 
been submerged; on them are found, too, survivals of the Bassian 
fauna, and probably the flora, both living and extinct. The lower 
reaches of the Tamar Major River were the first to be flooded 
from the Southern Ocean, and this flooding gradually inundated 
the middle reaches and its tributaries. Hedley (1913), discussing 
the marine molliisca of the waters of south-eastern Australia, 
noted the decided difference between the faunas east and west 
of the Bassian Isthmus, and proposed the terms Peronian and 
Adelaidean for them. Considerable interchange has, he states, 
taken place since the Bassian Isthmus was breached, accelerated 
by the westerly winds and currents. While these are partly 
responsible, the fact that the western end of the Bass Strait 
Sunkland was flooded first is the main reason. Hedley did not 
visualise the Port Phillip Sunkland or the King Island Isthmus. 
Althougli the Bassian Isthmus seems to have been a land 
connection of considerable width and elevation — the highest peak 
in the Purneaux Group rises to a height of 2,550 feet and the 
last to be breached, it has, so far, yielded few fossil remains of 
mammals. Most of the evidence of the fossil fauna comes from 
the King Island Isthmus — from King Island itself, or from 
Mowhray Swamp in Tasmania, at the southern end of the 
