BRITISH OYSTERS: OLD AND NEW. 185
shells are plentiful and usually in fine condition, as in the case of
other genera.
M. Sars (Edin. New Phil. Mag. 1863) records the oyster from
the Glacial Beds of S. Norway, at Kellebo, in Rakkestaat, at
300—440' elevation. In the Christiania region, Brgger makes 
the great Oyster banks the earliest post-glacial deposits;
these and the succeeding Tapes banks are possibly
the equivalent of our Irish Estuarine Clays, as well as those
of the Nar Valley and Grangemouth. Oyen gives Trondjhem
for its farthest appearance northwards in a post-glacial deposit,
where specimens are found 105 mm. long. Thick masses of
shells abound on the shores of the Cattegat, where the oyster
flourished during Neolithic times, as the hundreds of shell-
heaps testify, some of them being banks 1,000 feet in length,
by 80 to 150 feet in breadth. The shells are of the usual size,
but, owing to the influx of fresh water from the Baltic during
the Ancylus-sea period, died out, as the mollusc cannot exist
in water containing less than 16 or 17 parts in 1,000 of saline
matter. (Sir H. Howorth, Geol. Mag. 1905, p. 461). Forchammer
[Trans. Geol. Soc., vol. v. (1837-40) p. 159] reports that
in Holstein at 150 feet above sea level a bed of pebbles with
Cardium edule, Buccinum undatum and Ostrea edulis occurred,
the oysters being much smaller than those now living on the
Coast.
Thanks to the courtesy of Dr. Odhner of Stockholm, Dr.
Oyen, of Christiania, and Prof. Ravn, of Copenhagen, I am
now in possession of a fair series of oysters from Norway, Sweden
and Denmark, including Uddevalla, Bohuslan (recent and fossil),
the Danish Kjokkenmoddings, and the Limfiord, in the North
of Jutland. The Uddevalla shell in some respects approaches
the Irish estuarine form in the incurving of the upper valve, but
it has much more exuberant foliation or fluting on the lower
valve where the growth-lines intersect the costs, and the valves
are slighter in texture with more delicate laminar growth at
the margin of the shell. The Uddevalla shell has the laminae
of the top valve set very close, not projecting beyond the plain
margins; lower valve with well-developed costae, rising with
hollow ridges where intersected by the annual shoots or growth-
marks. The midden-heap shells approximate to the Celtica
group in the strength of the costae. One type of the recent