A peculiarity in the skin of the true C. adapts them for their manner of life. The skin is extremely thick, the inner part of it consisting of elastic fibers interlacing each other in every direction, the interstices of which are filled with oil, forming the sub stance called blubber. The oil deposited in this unusual situation, not only serves the ordinary purposes of fat, but that also of keeping the body warm, which to a. warm-blooded animal, continually surrounded with water, is of great importance; whilst the elasticity of this extraordinary skin affords protection in the great depths to which some of the whales descend, and in which the pressure must sometimes amount to a ton on every square inch.
The number of known species of C. is not great, but their natural history has as vet been very imperfectly studied. All of them are large animals, some of them by far the largest that now exist. Almost all of them—both herbivorous and ordinary—are marine, but some of the smaller species ascend large rivers to a great distance from the sea; and one, of the family d,elphinidoe, belongs exclusively to fresh waters, being found only in the upper tributaries of the Amazon and the elevated lakes of Peru.
Fossil cetacea have been hitherto discovered only in the tertiary formation. Their
remains represent species not only belonging to each of the recent families of true C., but have supplied materials for forming a new family intermediate between the true whales and the herbivorous cetacea. These fossils were originally described as reptiles; but they have been satisfactorily shown to be carnivorous C. by Owen, who, from their remarkable conjugate teeth, has given the typical genus the name of zeuglodon (q.v.), and the family that of zeuglodontidtr. In all, six or seven species have been described belonging to this family, from the eocene and miocene beds of Europe and America. The delphinidte appears first in the miocene strata, and continue through the newer beds. The remains of a narwhal, which cannot be distinguished from the living species, have been found in several places in England. Of Physeterithe, three species have been noticed in pleiocene and pleistocene strata, belonging to the recent genus physeter. Fossil baltenidte occur in the miocenc and newer beds. Only four species have been described, if we exclude cetotolites (q.v.), a name given to teeth and ear bones, belonging to animals of this family, which occur in great numbers in the Suffolk Crag.