^L. lignorum^, the gribble. This little isopod is very destructive in its habits, boring into submerged woodwork, like the ship-worm (Teredo), and doing great damage to the piles of wharves, etc. It burrows to the depth of half an inch, and completely honeycombs the surface of the wood, which then scales off or rapidly decays, and is washed away by the waves. The animal then begins anew, and in a short time, or at the rate of about an inch a year, piles diminish in diameter where /Limnoria/ is plentiful, and are finally destroyed unless protected from these little crustaceans by a sheathing of metal. Although considered as pests, they have an economic value not so often recognized, as they attack all driftwood, and in time entirely destroy floating and water-logged [pg293] timber, which without their destructive agency might become serious obstructions to navigation. /Limnoria/ is only the fifth of an inch in length, and its back is covered with minute hairs, to which dirt usually adheres. It burrows with its mandibles, or jaws, which are chisel-like at the ends. Its habitat extends from a little above to a little below tide-marks throughout the whole Atlantic coast. GENUS ^Sphæroma^ ^S. quadridentatum.^ The name of the genus to which this species belongs is derived from the peculiar habit of many of the species of rolling themselves into a ball when alarmed. The body is so constructed as to admit of this singular change of shape. The abdomen turns under, and the last abdominal appendages, together with the narrow epimera, shut in the legs and cover the ventral portions with armor. The body, when extended, is an ellipse a little over a quarter of an inch in length and half that in breadth. The legs are hairy and adapted to walking. The anterior abdominal segments are fused into one, but are marked at the sides with depressed lines. The abdominal feet are plate-like and fringed with hairs. A slight elevation runs around the margins of the animal like a border. The color is variable. Some are a uniform slaty gray; others are marked with a longitudinal patch of color on the back. It is found from Cape Cod to Florida among algæ or rocks, and is easily recognized by its habit of rolling itself into a ball. ^Sphæroma destructor^ is a boring isopod, larger than /Limnoria/, and is even more destructive, since its holes are larger; but its range is limited, or, at least, it has so far been found only in the St. Johns River, Florida. [Illustration: /Sphæroma quadridentatum./] GENUS ^Idotea^ [Illustration: /Idotea marina./]