Whenever the comparative anatomy of mammalia shall be exhaustively treated, the structure of the existing species of elephants will claim a principal share of attention. They are so distinctly separated from other quadrupeds that an order is required for their sole reception. Yet, though isolated from all other existing species, they bear marks of affinity with more than one group, particularly with Ungulata, Sirenia, and Rodentia. This fact of recent zoology suggests a view which the history of extinct mammalia seems to justify, viz., that the elephants constitute a comparatively primitive type, representing, though perhaps not directly, a now extinct central group, from which the orders above mentioned, and possibly some others, may have been derived. The African and Indian elephants are further remarkable as the largest land quadrupeds now living ; nor can the palaeontologist, reviewing the land quadrupeds of all past times, so far as they are yet made known, find a trace of any larger form which is not also an elephant. Hence arise some curious physiological and mechanical questions to the student of elephant anatomy.