Arkell.1956.Jurassic..

596 GENERAL SURVEY
feet of birds, either as eggs, or larvae, or small imagos. A striking instance
of this is the lake Birket Qarun, in the Faiyum, Egypt, which has been fed
by a branch from the Nile and has contained the normal Nile fauna of
Corbicula and other molluscs for tens of thousands of years, as may be
seen by the abundance of shells in its high Pleistocene beaches, in which
they are associated with Palaeolithic implements. During the early years
of the present century the need for more cultivable ground led to artificial
shrinkage of the lake to a point where salinity suddenly made the water
suitable for marine or brackish mollusca. A survey of the fauna in 1907
made no mention of any marine forms. By 1927, however, the dominant
mollusca in the lake were Cardium edule and Scrobicularia cottardi, two
forms characteristic of the saline lakes near Alexandria (Gardner, 1932,
p. 84). They were so abundant by 1927 that their shells formed a continuous
beach mound for many miles along the lake shore (Sandford &
Arkell, 1929, pi. xi). More than 100 miles of dry desert separate the
Alexandrian lakes from the Birket Qarun, but there is a constant traffic
of duck between the two, and the lakes are visited by flocks of flamingos.
There can be no doubt that the mollusca were in some way transported
by these birds, probably in mud adhering to their legs.
In Jurassic times air-transport was perhaps more restricted, but with
Archaeopteryx and flying reptiles on the scenes, it cannot be affirmed as
impossible that marine organisms could have been carried across isthmuses
by air. And as Simpson has shown, anything that was not impossible
becomes a fair probability when there is so much credit to draw on in
the bank of time.
However this may be, we shall here attempt a review of some of the
features of distribution of fossil marine faunas which have a bearing on
migration routes and the extent of the oceans in at least one geological
period.
As a preliminary it should be stated that at no time in the Jurassic
were marine transgressions over the continents so extensive as to warrant
the assumption that the main oceans could have been dry land, least of all
the Pacific. Maps (e.g. Haug, 1900; Gregory, 1930, p. xci) which show
the Pacific as mainly dry land, on which the water is reduced to narrow
strips, necessarily imply a compensating inundation of the continents
of an order far greater than is warranted by the known distribution and
facies of the sediments. Even the greatest transgressions, such as that of
the Upper Jurassic across Russia, from the Arctic Ocean to the Baltic
and Caspian, are negligible compared to the effects of emptying the
Pacific. Furthermore, such transgressive seas were always shallow, as is
apparent from the sediments and faunas they left behind. Nor were they
contemporary everywhere. The Liassic period, for which Gregory
shows the Pacific almost drained of its water, was a period of regression
compared with the Upper Jurassic; a truth already pointed out threequarters
of a century ago by Neumayr.
We cannot get rid of the water in the oceans by postulating enlarged
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