Occasionally we find forms which occurring in a set of beds _A_ in an area, are absent from the overlying beds _B_, and appear again in the succeeding deposits _C_. Such cases of _recurrence_ are by no means rare, though many supposed instances of recurrence have been recorded as the result of stratigraphical or palæontological errors. The best examples have been noted by Barrande among the Lower Palæozoic deposits of Bohemia. The stage _D_ of Bohemia consists of five 'bandes' or subdivisions, the lowest (_d_ 1), central (_d_ 3) and uppermost (_d_ 5) divisions are mainly argillaceous, whilst the second (_d_ 2) and fourth (_d_ 4) are essentially arenaceous. Some of the forms found in _d_ 1, _d_ 3 and _d_ 5 have not been found in _d_ 2 and _d_ 4. The best-known example is the trilobite _Æglina rediviva_. It is clear that this and other forms did not become extinct during the deposition of the strata of _d_ 2 and _d_ 4, though they may have disappeared temporarily from the Bohemian area, or else lingered on in such diminished numbers that their remains have not been discovered. The range of the organism is in fact right through the deposits of the stage _D_, and the discontinuity of distribution is not a real anomaly; it may be compared to some extent with cases of discontinuous distribution in space. It is needless to remark that the whole fauna does not disappear for a time and then reappear, but only a few out of the many forms which compose it. The comparative rarity of examples of recurrence after long intervals is an indication that the palæontological record as it is termed is not so imperfect as some suppose, for if our knowledge of fossils were very imperfect, we should expect cases of apparent recurrence to be common, as the result of the non-detection of fossils in the intermediate beds. One of the most marked cases of apparent recurrence known some years ago was the reappearance of a genus of trilobite _Ampyx_ in Ludlow rocks, found in the Bala rocks, but not in the Llandovery or Wenlock strata. It has since been discovered in Llandovery beds, and its eventual discovery in beds of Wenlock age may be regarded as certain. A supposed case of recurrence which would have been remarkable, that of the disappearance of _Phillipsia_ in Ordovician rocks, its entire absence in those of Silurian age, and its reappearance in the Devonian, has broken down, for the supposed Ordovician form has been shown to belong to an entirely different group of trilobites from that containing the genus _Phillipsia_, and it has been therefore renamed _Phillipsinella_.