in zoology; a genus of apterous insects, the characters of which are: They have six feet formed for running; the mouth is furnished with four palpi, two of which are cætaceous and two capitated; the tail is terminated by extended bristles, and the body imbricated with scales. There are 7 species. The fasciaria (Plate CCLXXIV.) is an American species, so called because mostly found among sugar; but now common in Europe. It is of a leaden colour, but rather inclining to that of silver, by reason of the small silvery scales with which it is covered; by which same circumstance it resembles, especially in its under part, the silver fish. It is found in gardens, under boxes, and in the crevices of window-sashes in houses, where it is very common. It runs with great swiftness, and is difficult to catch. When touched, it loses part of its scales, and its softness makes it easy to crush.

LEPTUM, in natural history; a genus of fossils of the harder gypsum, composed of very small particles, and of a less glittering hue.

There is only one species of this genus, being one of the least valuable and most impure of the class of gypsums. It is of an extremely rude, irregular, coarse, and unequal structure; a little soft to the touch, of a very dull appearance, and of different degrees of a greyish white. It is burnt in plaster for the coarser works; it calcines very slowly and unequally, and makes but a very coarse and ordinary platter.