It must be distinctly understood that the determination of fossils as characteristic of different periods is solely made as the result of experience. No _à priori_ reasoning may give one indication of the actual range in time of a species or genus; no one can say why _Discina_ has a long range in time, whilst that of the closely related _Trematis_ is very limited. This being the case, the greater the mass of evidence which is accumulated as to the range of a fossil, the greater will be the value of that fossil as a clue to the age of the deposit in which it is found. This is so important, that it requires more than mere notice. If a fossil is found in abundance in a group of strata _B_ in any one area, and is not found in an underlying group _A_ or overlying group _C_ in that area after prolonged search, we may confidently speak of the fossil as characteristic of the strata _B_ in that area, though without further work, the value of the fossil as a clue to age in other areas would be unproved. It may nevertheless happen, that after more prolonged search in _A_ or _C_, in the original area a few specimens of the fossil which has been spoken of as characteristic of _B_ may be found in one or other of them, in small quantity. The value of the fossil as one characteristic of _B_ will be slightly diminished, though only slightly, as it is not likely to turn up in numbers in the strata _A_ or _C_ after the prolonged search. Should the fossil be found also to be characteristic of the strata _B_ in areas other than the original one, it becomes of more than local value, and if, after much study it is found to characterise the same strata over wide areas, the cumulative evidence now obtained will render the fossil peculiarly important to the stratigraphical geologist. The detection of characteristic fossils is not quite so simple as might be supposed from the above remarks, for examination of the position of one fossil will not prove the contemporaneity of beds in different places, to prove this, all the evidence at our disposal must be considered, for reasons which will be presently pointed out. As the result of accumulated knowledge, we can now compile lists of characteristic fossils of the major subdivisions of the strata, which are of world-wide utility and as our knowledge increases, we are enabled to subdivide the strata into minor divisions of more than local value.