are immersed in a very little acid, they become more transparent, look like ghosts, and are soon lost to view; but if much acid is added, they disappear instantly. After a very large number have been dissolved, a flocculent residue is left, which apparently consists of the delicate ruptured cell-walls. In the two posterior pairs of glands the carbonate of lime contained in the cells occasionally aggregates into small rhombic crystals or into concretions, which lie between the lamellæ; but I have seen only one, and Claparède only a very few such cases.
The two anterior glands differ a little in shape from the four posterior ones, by being more oval. They differ also conspicuously in generally containing several small, or two or three larger, or a single very large concretion of carbonate of lime, as much as 1½ mm. in diameter. When a gland includes only a few very small concretions, or, as sometimes happens, none at all, it is easily overlooked. The large concretions are round or oval, and exteriorly almost smooth. One was found which filled up not only the whole gland, as is often the case, but its neck; so that it
