THE BRITISH WOODLICE. 39
which has been named Archaeoniscus brodiei, and is said to be
referable to the recent family Aegidae is found in some numbers
in the Purbeck Beds (Upper Jurassic), of this country (47).
Fossil Isopods have also been recorded from the Oolite and
from the Oligocene (Isle of Wight).
Turning to the Woodlice proper, we find that they first make
their appearance in the Miocene (of Oenigen and Baden), and
occur also in amber (79) ; while examples of the genera, such as
Oniscus and Porcellio, have been discovered in late Tertiary
deposits (47).
External structure and appendages.—Woodlice agree
in being of a somewhat oval form, and their bodies are arched,
the curve varying in different genera and species. A head is to
be distinguished ; behind this comes the thorax of seven segments
which are often considerably broader than the six succeeding
ones which form the abdomen (see fig. 1.)
The head carries two large antennae (fig. 3)
which are very evident, and a careful search
with a lens will reveal a second and minute
pair (the smaller antennae) situated between the
base of the others, and really anterior to them.
(figs. 2 and 4.)
The larger antennae are customarily bent
at certain points, and we can distinguish a