Dentine, enamel and cement are all represented in the teeth of fishes, but the enamel is generally very thin, and cement is but rarely developed. Dentine forms the main bulk of the teeth; it is sometimes of the normal type, but generally differs from that in higher vertebrates in being vascular, and is known as _vasodentine_. A third type occurs, known as _osteodentine_; it is traversed by canals occupied by marrow, and is closely allied to bone. [Illustration FIG. 15. PART OF THE LOWER JAW OF A SHARK (_Galeus_) (from OWEN after ANDRÉ). 1. teeth in use. 2. reserve teeth folded back. 3. part of the caudal spine of a Sting-ray (_Trygon_) which has pierced the jaw and affected the growth of the teeth.] The teeth are generally continually renewed throughout life, but sometimes one set persists. The teeth of Selachii are fundamentally identical with placoid scales. They are developed from a layer of dental germs which occurs all over the surface of the skin, except in the region of the lips. At this point the layer of tooth-producing germs extends back into the mouth, being projected by a fold of the mucous membrane (fig. 14, 7). Here new teeth are successively formed, and as they grow each is gradually brought into a position to take the place of its predecessor by the shifting outwards of the gum over the jaw. Owing to this arrangement sharks have practically an unlimited supply of teeth (figs. 14 and 15). Two principal types of teeth are found in ELASMOBRANCHS. In Sharks and Dogfish, on the one hand, the teeth are very numerous, simple, and sharp-pointed, and are with or without serrations and lateral cusps. Many Rays and fossil Elasmobranchs, on the other hand, have broad flattened teeth adapted for crushing shells. Intermediate conditions occur between these two extremes. Thus in _Cestracion_ and many extinct sharks, such as _Acrodus_, while the median teeth are sharp, the lateral teeth are more or less flattened and adapted for crushing. In various species belonging to the genus _Raia_ the teeth of the male are sharp, while those of the female are blunt. A very specialised dentition is met with in the Eagle-rays (Myliobatidae), in which the jaws are armed with flattened angular tooth-plates, arranged in seven rows, forming a compact pavement; the plates of the middle row are very wide and rectangular, those of the other rows are much smaller and hexagonal. Lastly, in _Cochliodus_ the individual crushing teeth are fused, forming two pairs of spirally-coiled dental plates on each side of each jaw. _Pristis_, the Saw-fish, has a long flat cartilaginous snout, bearing a double row of persistently-growing teeth planted in sockets along its sides. Each tooth consists of a number of parallel dentinal columns, united at the base, but elsewhere distinct.