Zeuglodon (Gr. A Yoke And Bdobc, A Tooth), a gigantic fossil cetacean mammal, found in the eocene and miocene tertiary strata of the southern United States and Europe, so named by Owen from the yoke-like character displayed by a section of the molar teeth. Its remains were first discovered in 1832 in the tertiary of Louisiana, and were supposed to belong to some huge saurian reptile, to which Dr. Harlan gave the name of basilosaurus; he carried the bones to London in 1839, where Owen showed by microscopic examination of the teeth, and the fact that the molars were double-rooted and implanted in double sockets, that it was not a reptile but a cetacean mammal, and belonged somewhere near the manatee and dugong. In 1835 Prof. Agassiz established the genus phocodon, from the examination of a tooth in the museum of Cambridge, England, regarding its possessor as nearly allied to the seal family; this was the very specimen figured by Scilla in 1747, in his work De Corporibus Marinis, and was obtained from the miocene of Malta; if phocodon be a synonyme of zeuglodon, the former has a priority of four years over the latter, and according to the rules of scientific nomenclature should be adopted, and with the more reason as the animal in question bears affinities to the seals in more respects than in the form of the teeth.

In 1840 M. Grateloup described the fragment of an upper jaw with teeth found in the eocene of France, a few leagues south of Bordeaux, which he believed to characterize a new order of amphibious reptiles, carnivorous and marine, perhaps a connecting link between the lacertians and the sharks, and for which he proposed the name of squalodon. In 1845 Dr. E. W. Gibbes described some teeth which he referred to a genus called by him dorudon, now recognized as belonging to the zeuglodon. The materials for the study of this animal have been extensively collected, though its exact position in the scale of mammals is not established beyond dispute. In 1843 Mr. Buckley found a considerable series of bones of zeuglodon in Clark co., Ala.; they consisted of a chain of 40 vertebrae, with a portion of the skull and lower jaw, a perfect humerus, and a few other bones, measuring in total length about 70 ft.; some of the vertebrae are 18 in. long and 12 in. in diameter on the articulating surfaces, and many are nearly perfect; the specimen belongs to the collection of the late Dr. J. C. "Warren of Boston. About the same time Mr. Koch, a German collector, obtained from the marly limestone of Alabama a considerable quantity of these bones, which were put together, embracing parts of different skeletons, and exhibited in most of the northern cities as the hydrarchus Sillimani, or great marine serpent.

Prof. "Wyman and others questioned the authenticity of this collection, which was carried to Dresden, and there described by Carus as a reptile, though Barraeister and Muller maintained that it belonged to a mammal. It was afterward purchased by the king of Prussia for the palseontological museum in Berlin. There is a specimen in the Boston society of natural history, containing 36 vertebras and 26 fragments of ribs and other bones. From all these sources it is known that the cranium was much elongated, and narrowed behind the frontal bones; the occipital region much and steeply elevated, as in the hog; frontals very wide above the orbits; face slender, with elongated nasal bones, and normal nasal openings unlike those of the cetaceans; intermaxillaries long and slender, and the lower jaw resembling that of the dolphins and sperm whales; the occipital condyles are two as in mammals, and the squamous cranial sutures and bones of the ear as in cetaceans. The cervical vertebras are very short; the dorsals elongated, with small spinous and transverse processes, the former consolidated to the cylindrical bodies, but not contiguous; their epiphyses are several inches thick, whereas in the cetaceans in bones of equal size they form plates not more than half an inch thick; the bodies of the caudal vertebras are very long; the ribs are short, of a dense laminated structure, somewhat thickened at the lower extremities, as in the sirenia; bones of arm small, the distal end of the humerus being suddenly contracted, and having the articulating surface of a hinge-like joint.

The form was probably cetacean, though slender, elongated, and more snake-like, with small anterior limbs like paddles, and no posterior extremities; from the long chain and characters of the vertebras, and the shortness of the ribs, as figured by Pictet, it seems that the spinal column must have been freely movable in its several parts, presenting no anatomical impossibility to the zeuglodon's performing the part of a tertiary and perhaps modern sea serpent. (See Sea Serpent.) The dentition is peculiar; in the largest and best known specios (zeuglodon cetoides, Owen) the formula is: incisors £-£, abnormal canines 1/0-1/0, and molars 5/5 - 5/5 = 36; the anterior teeth are conical, the molars being compressed, serrated on the edges, and doublerooted; the interval between the long roots of the molars is continued by a marked depression on the sides of the crown, so that when the teeth are much worn each presents two surfaces united by a thin connecting substance, whence the name given by Owen; the prior name of phocodon of Agassiz is just as proper on account of the serrations of the cutting edges, like those seen in many seals, to which family some authors have approximated it.

The dentition of the zeuglodon and the nasal openings prove that it was not a typical cetacean, and the molars and shape of the head also remove it from the dngong and manatee; yet the affinities seem nearer to these aquatic types than to any other. In the mode of completion and succession of the teeth, according to Owen, it belongs to a higher type than that of any of the existing cetaceans; he therefore regards it as an interesting link between these (sperm whale, dolphin, etc.) and the sirenia (dugong and manatee). - For fuller details and plates, besides the works quoted, see "Journal of the Academy of Natural Sciences of Philadelphia," 2d series, vol. i. (1847), pp. 5-17, including papers by Messrs. Gibbes and Tuomey. Several species are described.