INTRODUCTION. Definition of the Term ‘Bajocian.’ —For the strata which are equivalent to the upper part of the Inferior Oolite, with a portion of the Fullers' Earth, d'Orbigny proposed the term ‘ Bajocien . ’ Like our own terms ‘ Inferior Oolite ’ and ‘ Fullers ’ Earth,’ its boundaries were somewhat uncertain; and like them, too, it would seem that the same palæontological horizon has received different geological names at different localities—for this, probably, a difference in lithological characters may be blamed. In the present paper I use the term ‘ Bajocian ’ in a merely conventional sense—for the lower beds of the upper part of the ‘ Inferior Oolite ’; but I do not express any opinion as to its merits. I may, however, remark that the division of the Jurassic period on palæontological grounds— disregarding the details of its inconstant and merely local lithology —is a necessity. Ammonites have been chosen as the indicators of horizons, and their rapidity in development makes them peculiarly suitable. Therefore, as far as possible, the chronological unit and the Ammonite–species should go together; and any system of grouping the chronological units should depend on the epacme, acme, and paracme of Ammonite-families. No doubt, in practical application, the epacme of one family would be found contemporaneous with the paracme of another, so that possibly it might be necessary to consider only two of the developmental phases. Such terms as Bajocian, Toarcian, etc., might be used from the chronological point of view only, to