As mentioned earlier, most amateur collectors collect very few vertebrate remains, and for this reason this group is not discussed in detail. However, the more important vertebrate classes are briefly reviewed to enable the reader to have some understanding of this important group of animals. This part of the handbook will also serve as an introduction to some of the interesting and unusual, but now extinct, animals that have inhabited Texas in the geologic past. Among these animals are giant fishes, primitive amphibians, and many different types of dinosaurs. Included also are such unusual mammals as the giant ground sloths, saber-tooth cats, mammoths, and mastodons, all of which are now extinct. The remains of these, and many other interesting extinct vertebrates, may be seen in the geological collections of the Texas Memorial Museum at Austin. Many of these displays are accompanied by drawings which depict the scientific restoration of the animal’s soft parts and show how the animal may have appeared in life. Superclass Pisces The members of this superclass are commonly called fishes and are the simplest and most numerous of all vertebrates. They are aquatic, free-moving, cold-blooded (their blood maintains the temperature of the surrounding water), and breathe primarily by means of gills. However, some forms (the lungfishes) breathe by means of a lung developed from the air-bladder. The most recent fish classification recognizes four major classes, the Agnatha (primitive jawless fishes), the Placodermi (armored fishes with primitive jaws), the Chondrichthyes (sharks and related forms with cartilaginous internal skeletons), and the Osteichthyes (true bony fishes). CLASS AGNATHA.— Fishes belonging to this class are primitive, jawless, and represented by the living lampreys and hagfishes. The first agnathans appeared in the Ordovician and were armored by a bony covering on the front part of their bodies. These primitive fishes, called _ostracoderms_, are the earliest recorded fishes and, in addition, appear to be the first known vertebrate animals. The ostracoderms first appeared in late Ordovician time, increased in numbers in the Silurian, and were extinct by the end of the Devonian. CLASS PLACODERMI.— These are primitive jaw-bearing fishes, the majority of which were heavily armored (Pl. 37). The _placoderms_ were shark-like in appearance, and some of them grew to be as much as 30 feet in length. Members of this class appeared first in the Devonian and lasted into the Permian, at which time they became extinct. Placoderms are rare in Texas, but the fragmentary remains of these primitive fishes have been found in Devonian rocks in central Texas. CLASS CHONDRICHTHYES.—