The mouth of fishes either opens from beneath, as in the rays, or at the extremity of the muzzle, as in the great majority of species, or from the upper surface, as in a small foreign group called _Uranoscopus_, or moon-gazer—an odd name for species, some of which have been alleged to bury themselves to the depth of twenty feet in sand—a bed not easily obtained, and in no way fitted for astronomical observation. It also varies much in its relative dimensions, from the minute perforation of the genus _Centriscus_, to the vast expanded gape of the ugly angler-fish. We mean nothing personal in the last allusion. The teeth of fishes are frequently very numerous, and are sometimes spread over all the bony parts of the interior cavity of the mouth and pharynx, that is, on the maxillary, inter-maxillary and palatal bones, on the vomer, tongue, branchial arches, and pharyngeal bones. In certain genera they exist on all those parts; while in others they are wanting on some, or are even entirely absent on all. The denominations of the teeth are derived from their position, that is from the bones to which they are attached, and are consequently as numerous as the varieties of their situation. In the upper portion of the mouth of a trout, for example, there are five rows of teeth. The single middle-row is placed upon the central bone of the mouth called the _vomer_; a row on each side of it is fixed on the right and left _palatal_ bones, while the outer-rows or those of the upper-jaw, properly so called, are situate on the _maxillary_ bones. In the under portion of the mouth there are four rows, that is, one on each side of the tongue, and another external to these on each side of the lower-jaw. As to the form of teeth in fishes, the majority are hooked and conical, and more or less acute. In the majority of osseous fishes, besides the lips, which, even when fleshy, having no peculiar muscles, can exert but little strength in retaining the aliments, there is generally in the inside of each jaw, behind the anterior teeth, a kind of membranous fold or valvule, formed by a replication of the interior skin, and directed backwards, of which the effect is to hinder the alimentary substances, and especially the water gulped during respiration, from escaping again by the mouth. This structure does not, as formerly supposed, constitute a character restricted to the genus _Zeus_, but exists in an infinity of fishes.