It is interesting to note that all the fossil fish which are discovered from time to time have either ganoid or placoid scales, a fact which shows that the sharks, rays and sturgeon are directly descended from creatures which swam the seas thousands of years ago. The shells of shell-fish are not easy to examine microscopically, but frequently their plates may be detached from the edges of such shells as oysters and mussels and these should be examined. If the outer part of the shell be taken we can easily see its honeycomb structure and, by adding a little acid and waiting till all action has ceased, we shall have a structure remaining which is remarkably like a number of plant cells. The inner layer of many of these shells is composed of beautifully iridescent mother-of-pearl. Now such iridescence is usually caused by surfaces furrowed with many very fine lines and mother-of-pearl is no exception. Under the microscope, with a moderately high magnification, we can see minute striations all practically parallel to one another. The cuttlefish is peculiar in having a skeleton which is a moderately soft plate. These plates can often be found washed up by the tide, may be cut out from a dead cuttlefish or bought from a chemist’s as cuttlefish bone. However we secure the material we shall find that one side of the “bone” is hollow and that across this hollow, delicate plates run parallel to one another at intervals. Between these parallel plates there appear to be a number of fibres but, if we cut a thin slice of the structure and examine it under the microscope, we shall see that the apparent fibres are really very thin plates of bone which wind and double upon themselves in a beautiful manner. The structure of these plates gives strength to the bone without adding to its weight. Snails are sure to attract the microscopist sooner or later, so too are slugs. Many of the latter have shells, small flat or ear-shaped shells, quite different to the portable homes of snails. In many young snails, which may be killed by dropping into boiling water, we can find the shells so transparent that they form good objects for our microscope. Sometimes they are composed of six-sided cells, sometimes of beautiful star-shaped cells.