Scientists recognize two suborders of living cetaceans: the whalebone whales, suborder Mysticeti, and the toothed whales, suborder Odontoceti. The two groups are separated in the following ways:
BALEEN OR WHALEBONE WHALES. These animals are called whalebone whales because when fully formed instead of teeth they have up to 800 or more plates of baleen or whalebone depending from the roof of the mouth. They use these plates to strain their food, which consists of "krill" (primarily small crustaceans) and/or small schooling fish, by taking water into the mouth and forcing it out through the overlapping fringes of the baleen plates. Baleen whales are externally distinguishable from toothed whales by having paired blowholes. There are eight species of baleen whales in the western North Atlantic, ranging in size from the minke whale (just over 30 feet [about 9.1 m])[5] to the blue whale (85 feet [25.9 m]).
[5] Throughout this guide, measurements are given first in feet or inches, followed in parentheses by their equivalents in meters or centimeters. It is recognized that field estimates cannot be as precise as most of the conversions used.
TOOTHED WHALES. Unlike the baleen whales, the toothed whales do have teeth after birth. The teeth vary in number from 2 to over 250, though they may sometimes be concealed beneath the gum. In addition, toothed whales have only a single blowhole. This group includes the animals commonly called dolphin or porpoise as well as some commonly called whales (for example, the sperm whale). There are currently about 30 species of toothed whales known from the western North Atlantic, ranging in maximum adult size from the common or harbor porpoise, which is approximately 5 feet (1.5 m) long, up to the sperm whale which reaches a length of 68 feet (20.7 m). Several other species which are expected to be found in this region, though they have not yet been reported, are also included in this guide.
CLASSIFICATION OF CETACEANS
In addition to the two suborders (Mysticeti and Odontoceti), the cetacean order contains numerous families, genera, and species. Each of these groupings represents a progressively more specialized division of the animals into categories on the basis of similarities in their skulls, postcranial skeletons, and external characteristics. The discipline which concerns itself with naming an animal and assigning it to its appropriate scientific category is known as taxonomy. An example of the classification of a cetacean species is shown in the following:
SCIENTIFIC CLASSIFICATION OF THE ATLANTIC BOTTLENOSED DOLPHIN
| Kingdom: | Animalia | all animals |
| Phylum: | Chordata | having at some stage a notochord, the precursor of the backbone |
| Subphylum: | Vertebrata | animals with backbones—fishes, amphibians, reptiles, birds, and mammals |
| Class: | Mammalia | animals that suckle their young |
| Order: | Cetacea | carnivorous, wholly aquatic mammals: whales, including dolphins and porpoises |
| Suborder: | Odontoceti | toothed whales as distinguished from Mysticeti, the baleen whales |
| Family: | Delphinidae | dolphins |
| Genus: | Tursiops | bottlenosed dolphins |
| Species: | truncatus | Atlantic bottlenosed dolphin |
Modern taxonomy had its origin with the Swedish naturalist Linnaeus, whose tenth edition of the Systema Naturae in 1758 forms the official starting point. Following Linnaeus, modern scientific names consist of two words, a generic name, which has an initial capital, and a species name, which rarely does, occasionally in botany (some species names deriving from a person's name are capitalized). Both names are usually of Latin origin (sometimes Greek) and are italicized or underlined. These scientific names are of particular importance because, although common names of species often are different in different countries or even in different regions of the same country, the scientific name remains the same. For example, the right whale is universally known as Eubalaena glacialis though its common names include black right whale, nordcaper, sletbag, Biscay whale, and Biscayan right whale.
Although classification of many species is still in a state of flux, the classification of western North Atlantic cetaceans followed in this guide is as follows: