This class includes such modern forms as the sharks, rays, and skates. They are characterized by skeletons which are composed of cartilage and are very abundant in the marine waters of today. The earliest known representatives of this class are reported from rocks of Devonian age, and they have been relatively common up to the present time. Shark teeth (Pl. 37) can be found in Texas in Pennsylvanian, Permian, Cretaceous, Paleocene, Eocene, and Miocene rocks. These are probably the most common vertebrate fossils to be found in Texas and are usually found in thin-bedded marine limestones or clays. CLASS OSTEICHTHYES.— The Osteichthyes includes the true bony fishes, which are the most highly developed and abundant of all fishes. They possess an internal bony skeleton, well-developed jaws, an air-bladder, and, typically, an external covering of overlapping scales. Included in this class are a primitive group of fishes called _crossopterygians_. These were very abundant in the Devonian and are believed to be the ancestors of the amphibians. The modern lungfishes also belong to the class Osteichthyes, and these primitive fishes, which are now found only in Australia, South America, and Africa, breathe by means of gills and lungs which have been developed from the air-bladder. Although not abundant as fossils, the remains of these specialized fishes have added much to present knowledge concerning the development of certain of the higher vertebrates. The remains of bony fishes have been collected at many localities in Texas, and fossils of this type have been found primarily in rocks of Cretaceous age but have been reported from other rocks as well. Fish fossils are more commonly found in the form of teeth (Pl. 37), vertebrae, scales, and an occasional well-preserved skeleton. [Illustration: Plate 37] [Illustration: SHARK TEETH × 1] [Illustration: CONODONTS (GREATLY ENLARGED)] [Illustration: PRIMITIVE ARMORED FISH PLACODERM (DEVONIAN) × ½] _Conodonts_ (Pl. 37) are small, amber-colored, tooth-like fossils which are believed to represent the teeth of some type of extinct fish. Although geologists do not know a great deal about the origin of these strange fossils they are of value in micropaleontology. Conodonts have been reported from several Paleozoic formations in Texas and are useful guide fossils in some areas. Superclass Tetrapoda The tetrapods are the most advanced chordates and are typified by the presence of lungs, a three- or four-chambered heart, and paired appendages. Included here are the classes Amphibia (frogs, toads, and salamanders), Reptilia (lizards, snakes, turtles, and the extinct dinosaurs), Aves (birds), and Mammalia (including the mammals, such as men, dogs, whales, etc.).