According to the very varied habits, their external appearance varies within wide limits, there being amongst the 300 species, with 50 genera, arboreal, terrestrial, burrowing and semi-aquatic forms, and even one semi-marine kind. All have well-developed limbs. In their general structure the _Iguanidae_ closely resemble the _Agamidae_, from which they differ mainly by the pleurodont dentition. Most of them are insectivorous. Some, especially _Anolis_ and _Polychrus_, can change colour to a remarkable extent. The family ranges all through the neotropical region, inclusive of the Galapagos and the Antilles, into the southern and western states of North America. Remarkable cases of discontinuous distribution are _Chalarodon_ and _Hoplodon_ in Madagascar, and _Brachylophus fasciatus_ in the Fiji Islands. _Conolophus subcristatus_ and _Amblyrhynchus cristatus_ inhabit the Galapagos; the former feeds upon cactus and leaves, the latter is semi-marine, diving for the algae which grow below tide-marks. For _Basiliscus_ see BASILISK; IGUANA is dealt with under its own heading; allied is _Metopoceros cornutus_ of Hayti. _Polychrus_, the "chameleon," and _Liolaemus_ are South American; _Ctenosaura_ of Central America and Mexico resembles the agamoid _Uromastix_. _Corythophanes_ and _Laemanctus_, with only a few species, are rare inhabitants of the tropical forests of Central America and Mexico. _Sauromalus_, _Crotaphytus_, _Callisaurus_, _Holbrookia_, _Uma_, _Uta_ are typical Sonoran genera, some ranging from Oregon through Mexico. Allied is _Sceloporus_, with about 34 species, the most characteristic genus of Mexican lizards; only 4 species live in the United States, and only 3 or 4 are found south of the Isthmus of Tehuantepec and are restricted to Central America. The majority are humivagous, while others are truly arboreal, e.g. _S. microlepidotus_, a species which, moreover, has the greatest possible altitudinal range, from the hot country of southern Oaxaca to the upper tree-line of Citlaltepetl, about 13,500 ft. elevation; many species are viviparous. _Phrynosoma_, with about a dozen species, the "horned toads" of California to Texas, and through Mexico. Some of these comical-looking little creatures are viviparous, others deposit their eggs in the ground. They are well concealed by the colour of their upper parts, which in most cases agrees with the prevailing tone of their surroundings, mostly arid, stony or sandy localities; the large spikes on the head protect them from being swallowed by snakes. The enlarged spiny scales scattered over the back look as if it were sprinkled with the dried husks of seeds. They are entirely insectivorous, bask on the broiling hot sand and then can run fast enough; otherwise they are sluggish, dig themselves into the sand by a peculiar shuffling motion of the fringed