In attempting to adjust the scale of glacial periods to that provided by the succession of implement-forms, it is suggested that a commencement should be made by considering the period designated Mousterian. If the position of the Mousterian period can be correlated with a definite subdivision of the Ice Age, then other periods will fall into line almost mechanically. The first enquiry to make is that indicated in the introductory paragraphs of this Chapter, viz. what is the general nature of the fauna accompanying Mousterian implements? Investigation of the records shews that this is characteristically of a northern or a temperate, but not a southern type. For the combination commonly regarded as indicative of the southern type (viz. _Elephas antiquus_, _Rhinoceros merckii_, and _Hippopotamus major_) is very doubtfully demonstrable in this association, save in the very remarkable instance of the Grotte du Prince, Mentone, and Boule (1906) makes somewhat laboured efforts to explain this example, which is exceptional in his opinion. On the other hand, that combination does occur in well-recognised inter-glacial deposits, _e.g._ the Swiss Lignites of Dürnten, etc. The Mousterian implements commonly accompany much more definitely northern animal forms, so that a glacial rather than an inter-glacial age is indicated. But there are four such glacial phases from which to choose in Professor Penck's scheme, and in Professor Boule's scheme there are two (for the 'Pliocene glaciation,' appearing in the latter, is hardly in question). It will be seen (by reference to Table B) that Professor Boule assigns typical Mousterian implements to the most recent glacial period (Boule's No. III = Penck's No. IV = Würm), whereas Professor Penck places them in his penultimate grand period (Riss), carrying them down into the succeeding (Riss-Würmian) inter-glacial period. Much diligence has been shewn in the various attempts to decide between these, the two great alternatives. (The view of Professor Hoernes, who assigns the Mousterian types to the first inter-glacial period of Penck, has received so little support as to render it negligible here.) Upon an examination of the controversial literature, the award here given is in favour of Professor Boule's scheme. The following reasons for this decision deserve mention. (1) Almost the only point of accord between the rival schools of thought, consists in the recognition by each side that the Magdalenian culture is post-glacial. But beyond this, the two factions seem to agree that the Mousterian culture is 'centred' on a glacial period but that it probably began somewhat earlier and lasted rather longer than that glacial period, whichever it might be.