Gems, a closely allied group of animals or plants, characterized by ultimate structural peculiarities. Great confusion prevails among describers in the formation of genera, from their considering form and complication of structure as generic characters, whereas the former is characteristic of families, and the latter of orders; hence generic, family, and ordinal characters are mixed up in the determinations of almost all naturalists from Linna?us to the present time, and genera have been unnecessarily and absurdly multiplied. Genera are subdivisions of families, and species are subdivisions of genera; the former, as has been stated, are limited by ultimate structural peculiarities, while the latter bear a closer relation to each other and to their special localities, their existence being also confined within a definite period. Generic peculiarities extend to the most minute details of structure of teeth, hair, scales, cerebral convolutions, distribution of vessels, arrangement of intestinal folds and appendages, and microscopic anatomy of the organs; so complete is this identity of structure that (in the words of Agassiz, Essay on Classification," part i., chap. 1) if an animal were "submitted to the investigation of a skilful anatomist, after having been mutilated to such an extent that none of its specific characters could be recognized, vet not only its class, or its order, or its family, but even its genus, could be identified as precisely as if it were perfectly well preserved in all its parts." Every species of the genus vulpes (fox), for example, has the same dental formula, toes and claws, and other generic characters, whether arctic, tropic, or temperate, American, European, or Asiatic, in its habitat.

Genera may or may not resemble each other in form: they usually have a wide geographical range, and are less modified in their characters by physical and external agencies. The generic distinctions of the testudi-nata or tortoises, both land and marine, founded principally on the characters of the skull, jaws, skin, and feet (see Agassiz, Natural History of the United States," vol. i.), give an admirable idea of what constitutes a genus, as distinguished from families and orders.