22 
Legg on Sponge Sand. 
Australia, and other localities, afford ample proof of numerous 
and beautiful forms. 
After collecting these minute shells, two very important 
points are, mounting them for future examination, and viewing 
them so as to obtain their true structure. Much depends on 
the different genera, as to the most eligible mode ; the simplest 
and most natural is that adopted by Mr. Marshall of placing 
them in cells made of perforated card, putting a piece of thin 
glass over and sealing it down, so that the objects roll abo\it 
loosely, and are viewed as opaque objects by a side light; 
another mode, also adopted by the same gentleman, is placing 
the shells on a glass slip with a little very dilute gum water, 
which causes the shells to adhere sufficiently to the glass as 
to admit of the air being exhausted from them when mounted 
in Canada balsam. They may then be viewed either as opaque 
or transparent objects^ but it will be observed that the texture 
of the shell which invests the segments of the animal is (as 
observed by M. d'Orbigny) very variable, but it almost always 
follows the different mode of growth upon which the orders of 
that author are founded. When the segments are closely packed 
together, the shell is opaque, of a close texture like porcelain, 
and without any indications of external porosity ; when the 
segments are alternate without a spire, and when the spire is 
oblique, the shell is porous, and pierced over the last cells 
with a great number of little mouths, through which proceed 
the filaments, but which become obliterated when the animal 
no longer needs them ; when the segments are in a straight 
line, when they are coiled upon the same spiral plane, or 
when they are alternate, and the shell inequilateral, their 
texture is almost as transparent as glass. 
From the above description it will be evident that one 
single mode of illumination will not suffice for duly developing 
the structure of these shells, and I should therefore recommend 
that some be mounted loosely in a cell, so that all parts may 
be viewed as they roll over ; and others be mounted in Canada 
balsam, and viewed by means either of the annular condenser 
of Mr. Shadbolt or the parabolic reflector of Mr. Wenham. 
By these means the difference in structure between the upper 
and under surfaces of the same species of shell is brought out, 
and that confusion avoided which occurs when direct light is 
transmitted. 
