He introduced also useful distinctions between Special, General, and Serial Homology. "The relations of homology," he writes, "are of three kinds: the first is that above defined, viz., the correspondency of a part or organ, determined by its relative position and connections, with a part or organ in a different animal; the determination of which homology indicates that such animals are constructed on a common type; when, for example, the correspondence of the basilar process of the human occipital bone with the distinct bone called 'basi-occipital' in a fish or crocodile is shown, the _special homology_ of that process is determined. A higher relation of homology is that in which a part or series of parts stands to the fundamental or general type, and its enunciation involves and implies a knowledge of the type on which a natural group of animals, the Vertebrate, for example, is constructed. Thus when the basilar process of the human occipital bone is determined to be the 'centrum' or 'body' of the last cranial vertebra, its _general homology_ is enunciated. "If it be admitted that the general type of the vertebrate endoskeleton is rightly represented by the idea of a series of essentially similar segments succeeding each other longitudinally from one end of the body to the other, such segments being for the most part composed of pieces similar in number and arrangement, and though sometimes extremely modified for special functions, yet never so as to wholly mask their typical character--then any given part of one segment may be repeated in the rest of the series, just as one bone may be reproduced in the skeletons of different species, and this kind of repetition or representative relation in the segments of the same skeleton I call 'serial homology'" (p. 7). As an example of serial homology we might take the centra of the vertebrae--the vomer, the presphenoid, the basisphenoid, the basioccipital and the series of centra in the spinal column. Such serially repeated parts are called _homotypes_ (p. 8).