Plagiostomes (Gr. , transverse, and , mouth), a suborder of cartilaginous fishes, including the sharks and rays, in some respects the most highly organized of their class. They correspond to the selachians of Cuvier, and to the placoids of Agassiz. They are few in number in the present creation, but with the ganoids (sturgeons, etc.) were by far the most abundant in past geological epochs, these two suborders being the only members of the class found below the chalk. The centre of their vertebral column is usually more or less ossified and divided into separate vertebra, and even when it forms a continuous chorda dorsalis the divisions are indicated by transverse partitions; the skull is united to the spine by a joint with a conical cavity, and the former is a simple cartilaginous capsule, without sutures, having a separate cartilaginous arch which performs the office of upper jaw; the mouth is arched, very wide on the lower surface, and at some distance from the snout, which is much developed for the accommodation of the large nasal capsules; the teeth are in numerous rows, the inner coming forward to replace those worn away by use; the branchial sacs are separated, with five or more distinct openings on the sides or lower parts of the body; the gills consist of membranous folds or plane surfaces, without the pectinated arrangement of osseous fishes, and with a pseudo-branchia; there is no swimming bladder; the scapular arch is detached from the head, and the ventrals are abdominal; on the upper surface of the head, behind the eyes, in most, is a pair of spiracles, communicating with the pharynx; the skin is covered with hard rough grains or scattered spines; in the arterial bulb are from two to five transverse rows of semilunar valves; the intestine has a spiral valve; the optic nerves do not decussate, but are connected by a commissure, and there is no arterial plexus between the layers of the choroid coat of the eye.

The secreting reproductive organs communicate with the ureters, and end in a rudimentary process in the cloaca; the claspers are present in the males as appendages to the posterior edge of the anal fins, fissured toward the end, and communicating with a csecal subcutaneous sac, well lubricated with mucus, and are true intromittent organs; the ovaries are smaller than in osseous fishes, and the ova very few; some genera are viviparous, others oviparous, and others ovo-viviparous.