Mammalia constitute one of the five major classes of vertebrates (the others being Pisces, Amphibia, Reptilia, and Aves). There are, at latest count, some 4,170 species of mammals living today, classified in about 1010 genera, 130–140 families, and 18 orders (Corbet and Hill, 1980; Honacki et al., 1982). In addition, sixteen orders and uncounted genera and species of mammals are known to be extinct (see synoptic classification in Table 1). While most living mammals are terrestrial like their Mesozoic ancestors, one large progressive group has invaded the air (bats, represented by about 150 living genera and 920 species). Two other smaller groups are fully aquatic (whales, including some 40 genera and 80 species; and sirenians, 3 genera and 5 species). Mammals occupy all of the continents, and all of the larger oceanic islands. Faunal diversity is greatest in lowland tropical forests, woodlands, and savannas, decreasing rapidly at high elevations and latitudes.