[Illustration: Fig. 107.--The Black Scissors-bill (_Rynchops nigra_).] The Scissors-bills have received their name from the conformation of their beaks, which are flattened laterally into two laminæ fitting one on the other, forming two mandibles compressed into cutting blades, the upper being one-third shorter than the lower. In order to pick up the shrimps and small fishes on which they feed, these birds are obliged to skim the surface of the water, dipping the lower mandible of their bill, the upper mandible being kept open and clear of the water till aquatic insects or other small fry have entered into the lower portion of it.[25] The singular form of their bills is also of service to them in opening such bivalve shell-fish as come in their way. They frequently watch these mollusks, and when they notice that the shell of the latter is slightly open, they plunge the lower mandible of their long bill into it; they then break the ligament of the shell by beating it against the rock. The tenement being thus destroyed, there is no obstacle to their devouring the inhabitant. [25] Catesby says: "These birds frequent near the sea-coasts of Carolina. They fly close to the surface of the water, from which they seem to receive somewhat of food." The only remarkable species of this bird is the Black Scissors-bill (_Rynchops nigra_), frequently called the Cut-water. It is about the size of a Pigeon; its prevailing colour is white, top of the head and shoulders black, with a white band on the wings. These birds are very numerous in the West Indian seas. They fly with a slow motion, and, like the Gulls and other sea-birds, they occasionally form such dense flocks that the sky is actually darkened for the space of a league. GULLS AND ALLIED SPECIES. Gulls and their congeners include the well-known shore-birds generally called Gulls, more especially the Skuas (_Lestris_), Gulls (_Larus_), and Mews (_Gavia_). They are characterised by a light body, more or less compact; neck of moderate length; head ovate; bill shorter than the head, straight, compressed; convex ridge on upper mandible, nasal groove long; lower mandible with the angle long and narrow; mouth moderate; tongue fleshy; eyes small; legs generally short; tibia bare; tarsus short, compressed; hind toe small; middle toe longest; fore toe moderate in length and slender; connecting membrane full, margins only concave; claws generally small, arcuate, acute, and more or less compressed. These birds inhabit the sea-shore, along which they wander in search of food; the larger species preying on fish, crustacea and mollusca, and the carcasses of cetacea and other marine mammalia cast up by the sea. They all pursue shoals of fish in the open sea, often to great distances from land. Their plumage is full, soft, close, elastic, and well blended on the back and wings; wings long, broad, and pointed; the tail, of twelve feathers, rounded and forked.