Mr. Owen, in his _Homologies of the Vertebrate Skeleton_, has given in a Tabular Form his views of the homology of the bones of the head of vertebrates, and the names which he consequently proposes for each bone, with the synonyms as they occur in the writings of some of the most celebrated anatomical philosophers, Cuvier, Geoffroy, Hallmann, Meckel and Wagner, Agassiz and Soemmering. And he has added to this Table his reasons for dissenting from his predecessors {352} to the extent to which he has done so. He has done this, he says, only where nature seemed clearly to refuse her sanction to them; acting upon the maxim (our Aphorism X.) that new terms and changes of terms which are not needed in order to express truth, are to be avoided. The illustrations which I have there given, however, of this maxim, apply rather to the changes in nomenclature than in terminology; and though many considerations apply equally to these two subjects, there are some points in which the reasons differ in the two cases: especially in this point:--the names, both of genera and of species, in a system of nomenclature, may be derived from casual or arbitrary circumstances, as I have said in Aphorism XIII. But the terms of a scientific terminology ought to cohere as a system, and therefore should not commonly be derived from anything casual or arbitrary, but from some analogy or connexion. Hence it seems unadvisable to apply to bones terms derived from the names of persons, as _ossa wormiana_; or even from an accident in anatomical history, as _os innominatum_. It is further desirable that in establishing such a terminology, each bone should be designated by a single word, and not by a descriptive phrase, consisting of substantive and adjective. On this ground Mr. Owen proposes _presphenoid_ for _sphenöide anterieur_. So also _prefrontal_ is preferred to _anterior frontal_, and _postfrontal_ to _posterior frontal_. And the reason which he gives for this is worthy of being stated as an Aphorism, among those which should regulate this subject. I shall therefore state it thus: APHORISM XXIV. _It is advisable to substitute definite single names for descriptive phrases as better instruments of thought._ IT will be recollected by the reader that in the case of the Linnæan reform of the botanical nomenclature of species, this was one of the great improvements which was introduced. Again: some of the first of the terms which Mr. Owen proposes illustrate, and confirm by their manifest claim {353} to acceptance, a maxim which we stated as Aphorism XXII.: namely, When alterations in technical terms become necessary, it is desirable that the new term should contain in its form some memorial of the old one. Thus for 'basilaire,' which Cuvier exclusively applies to the 'pars basilaris' of the occiput, and which Geoffroy as exclusively applies (in birds) to the 'pars basilaris' of the sphenoid, Mr. Owen substitutes the term _basioccipital_.