especially in the older tertiary strata, than was indicated in the list published in 1833. A large number, in particular, of the forty-two species of Eocene testacea, to which the names of recent shells were given in the tables, cannot be considered as identical, if we adopt the same standard of specific distinctions as is recognized in the new edition of Lamarck's conchology, edited by M. Deshayes himself, in 1836. But although many corrections are indispensable, and the proportion of recent species found fossil in the Eocene, Miocene, and older Pliocene strata may be considerably less than was at first supposed, we have no reason on this account to feel discouraged in an attempt to found the classification and nomenclature of the tertiary periods on the great principle before explained; namely, the comparative resemblance of the testaceous fauna of each period to that of the neighbouring seas. There can be no cabalistic virtue in such numbers as 3. 17. or 40., which were at first imagined to express correctly the proportional number of identical species in three of the tertiary periods; but until the time arrives when we can obtain the general acquiescence of conchologists as to the real proportional numbers, we must endeavour to find some readier method of estimating the relation of one fauna to another; a method not involving the question of the identity or non identity of every fossil with some known recent species. Now, it has been suggested by Dr. Beck that, in order to form such an estimate of the comparative resemblance of the faunas of different eras, we may follow the same plan as would enable us to appreciate the amount of agreement or discrepancy between the faunas now existing in two distinct geographical regions. It is well known that, although nearly all the species of mollusca inhabiting the temperate zones on each side of the equator are distinct, yet the whole assemblage of species in one of these zones bears a striking analogy to that in the other, and differs in a corresponding manner from the tropical and arctic faunas. By what language can the zoologist express such points of agreement or disagreement, where the species are admitted to be distinct? In such cases it is necessary to mark the relative abundance in the two regions compared of certain families, genera, and sections of genera; the entire absence of some of these, the comparative strength of others, this strength being sometimes represented by the numbers of species, sometimes by the great abundance and size of the individuals of certain species. It is, moreover, important to estimate the total number of species inhabiting a given area; and also the average proportion of species to genera, as this differs materially according to climate. Thus, if we adopt comprehensive genera like those of Lamarck, we shall find, according to Dr. Beck, that, upon an average, there are, in arctic latitudes nearly as many genera as species; in the temperate regions, about three or four species to a genus; in the tropical, five or six species to a genus. The method of which the above sketch conveys but a faint outline, is the more easy of application to the tertiary deposits of Europe, because the conchological fauna of the Eocene period indicates a tropical climate; that of the Miocene strata, a climate bordering on the tropics; and that of the Older and Newer Pliocene deposits, a climate much more closely approaching to, if not the same as, that of the seas in corresponding latitudes. Although I cannot enter in this work into farther details, it may be stated that, if we compare tertiary formations on this principle, the nomenclature above proposed will not be inappropriate; for the fauna of the older, or Eocene, tertiary formations is still the first in the order of time in which there is an assemblage of testacea like that of the present ocean between the tropics ; and in this period a small proportion of mollusca are undistinguishable from living species; whereas at the opposite extreme of the series, or in the Newer Pliocene deposits, all conchologists agree that the marine shells are all, or nearly all, identical with those now inhabiting the nearest seas. As to the Miocene and Older Pliocene groups, the terms less and more will always express correctly the different degrees of analogy which their fossils bear to the assemblage of living species in similar latitudes. But it should never be forgotten that, as the extinct species preponderate in all groups, with the exception of the Newer Pliocene, it is from their characters that we derive the distinguishing feature in the palæontology of each period. The relative approach which the shells may make to the living fauna affords a useful and interesting term of comparison; but it is one feature only, and by no means the most prominent one, in the organic remains of successive periods. 292 CHAPTER XIV. RECENT AND TERTIARY FORMATIONS. Peru How to distinguish Recent from Tertiary strata - Recent and Newer Pliocene strata near Naples - near Stockholm and Christiania - in South America, on coasts of Chili and Rocks of Recent period, with human skeleton, in Guadaloupe Shells of living species, with extinct mammalia, in loess of the Rhine - Recent and Newer Pliocene deposits in England — Older Pliocene strata in EnglandCrag Red and Coralline crag - their fossils in part distinct — their strata unconformable-belong to the same period-London clay- Its shells and fish imply a tropical climate Tertiary mammalia-Fossil quadrumana. - RECENT and Newer Pliocene strata. If we begin with the history of the more modern aqueous formations, and then pass on to the more ancient, the first strata which present themselves are those termed, in the last chapter, the Newer Pliocene. But in what manner shall we define the limits between this group and those fossiliferous deposits which are now in progress, or which have accumulated under water since the globe was inhabited by man? The strata last mentioned, namely, those of the human period, I shall call Recent, distinguishing them from the most modern tertiary formations. Strata may be proved to belong to the Recent period by our finding in them the |