30 
CEXTEAL HALL. 
bivalve, multivalve, &c. ; then the nature of the outer layer, or 
jperiostramm ; and, thirdly, some of the more striking styles of 
ornamentation and coloration. Following these, a division is 
devoted to the display of the general form and to]30graphy of 
the shells of bivalves, especial attention being directed to the 
nature of the hinge by which the two valves are articulated. 
Treshwater mussels ( Unionidce) are selected as examples of great 
variability in the form of the shell in closely allied species. 
Near by is a small series of the shells of boring bivalves, many 
of them in the substances they perforate. 
The left half of the wall-case on the opposite side of this bay 
shows, in a similar manner, the varieties in form and structure 
presented by univalve gastropod shells. Many of the 
specimens have been cut to show the internal structure. The 
horny or shelly opercula closing the mouths of many Gastropods^ 
are also exhibited in this case. 
The right end of the same case displays spirit preparations 
of the Cephalopod Molluscs, such as Cuttle-fish, Squids, 
N'autilus, Paper-Nautilus, &c. Specimens of the horny 
beaks possessed by all members of this class are also 
exhibited ; and a Pearly ISTautilus, with the shell bisected in 
order to show the air-chambers and the comparatively small 
space occu])ied by the animal," will be found of special 
interest. 
The table-case in this bay, as mentioned above (p. 12), is 
devoted to the illustration of " Mimicry " and analogous pheno- 
mena. Many of the examples shown occur among insects, but 
a harmless snake, and a venomous species it mimics, are among 
the series. Very striking is a coloured sketch showing a group 
of red and black caterpillars aggregated side by side on the stem 
of a plant so as to present a remarkable similarity to a flower 
or seed -capsule. 
Bays VIII. Bay VIIL, on the eastern side of tlie Central Hall, is, as 
^° ^' already mentioned, temporarily occupied with illustrations of 
the recent volcanic eruptions in the West Indies. Bays IX. 
and X. are intended to illustrate the morpliology of the vege- 
table kingdom. 
I'hesebays are wholly devoted to the great group of Phanerogams 
or seed-plants, where, besides the distinction of stem, leaf, and 
