THE CLASSIFICATION OF FOSSIL ANIMALS AND PLANTS.

An elementary knowledge of the principles underlying the classification of animals and plants is essential to the beginner in the study of fossils.

The Naming of Animals.—

In order to make a clearly understood reference to an animal, or the remains of one, it is as necessary to give it a name as it is in the case of a person or a place. Before the time of Linnaeus (1707-1778), it was the custom to refer, for example, to a shell, in Latin[2] as “the little spiral shell, with cross markings and tubercles, like a ram’s horn;” or to a worm as “the rounded worm with an elevated back.” Improvements in this cumbersome method of naming were made by several of the earlier authors by shortening the description; but no strict rule was established until the tenth edition of Linnaeus’ “Systema Naturae” (1758), when that author instituted his binomial nomenclature by giving each form enumerated both a generic and specific name. In plain words, this method takes certain life-forms closely related, but differing in minute particulars, and places them together in a genus or kindred group. Thus the true dogs belong to the genus Canis, but since this group also includes wolves, jackals, and foxes, the various canine animals are respectively designated by a specific name; thus the dog (Canis familiaris), the dingo (C. dingo), the wolf (C. lupus), the jackal (C. aureus), and the fox (C. vulpes). The generic name is placed first. Allied genera are grouped in families, (for example, Canidae), these into orders (ex. Carnivora), the orders into classes (ex. Mammalia), and the classes into phyla or subkingdoms (ex. Vertebrata).

Plants are classified in much the same way, with the exception that families and orders are, by some authors, regarded as of equal value, or even reversed in value; and instead of the term phylum the name series is used.

Classification of the Animal Kingdom.

Classification of Animal Kingdom.

The first seven groups of the above classification are back-boneless animals or Invertebrata; the eighth division alone comprising the animals with a vertebra or backbone.

Characters of the Several Phyla.—

In the first group are placed those animals which, when living, consist of only one cell, or a series of similar cells, but where the cells were never combined to form tissues having special functions, as in the higher groups.

PROTOZOA.—

The Amoeba of freshwater ponds is an example of such, but owing to its skin or cortex being soft, and its consequent inability to be preserved, it does not concern us here. There are, however, certain marine animals of this simple type of the Protozoa which secrete carbonate of lime to form a chambered shell (Foraminifera); or silica to form a netted and concentrically coated shell held together with radial rods (Radiolaria); and both of these types are found abundantly as fossils. They are mainly microscopic, except in the case of the nummulites and a few other kinds of foraminifera, which are occasionally as large as a crown piece.

COELENTERATA.—

The second group, the Coelenterata, shows a decided advance in organisation, for the body is multicellular, and provided with a body-cavity which serves for circulation and digestion. The important divisions of this group, in which the organisms have hard parts capable of being fossilised, are the limy and flinty Sponges, the Corals, and allied groups, as well as the delicate Graptolites which often cover the surface of the older slates with their serrated, linear forms, resembling pieces of fret-saws.

ECHINODERMATA.—

The third group, Echinodermata, comprises the Sea-lilies (Crinoids), Starfishes and Sea-urchins, besides a few other less important types; and all these mentioned are found living at the present day. Their bodies are arranged in a radial manner, the skin being strengthened by spicules and hardened by limy deposits ultimately forming plates. They have a digestive canal and a circulatory system, and are thus one remove higher than the preceding group.

VERMES.—

The fourth group, Vermes (Worms), are animals with a bilateral or two-sided body, which is sometimes divided into segments, but without jointed appendages. Those which concern the student of fossils are the tube-making worms, the errant or wandering worms which form casts like the lob-worm, and the burrowing kinds whose crypts or dwellings become filled with solid material derived from the surrounding mud.

MOLLUSCOIDEA.—

Group five, the Molluscoidea, contains two types; the Flustras or Sea-mats (Polyzoa) and the Lamp-shells (Brachiopoda). They are at first sight totally unlike; for the first-named are colonies of compound animals, and the second are simple, and enclosed between two valves. They show in common, however, a bilateral symmetry. The mouth is furnished with fine tentacles, or with spirally rolled hair-like or ciliated processes.

MOLLUSCA.—

The sixth group, the Mollusca, includes all shell-fish. They are soft-bodied, bilaterally symmetrical animals, without definite segments. The shells, on account of being formed of carbonate of lime on an organic basis, are often found preserved in fossiliferous strata.

ARTHROPODA.—

The seventh group, the Arthropoda, or joint-footed animals, are distinguished by their segmented, lateral limbs, and by having a body composed of a series of segments or somites. The body and appendages are usually protected by a horny covering, the ‘exoskeleton.’ The group of the Trilobites played an important part in the first era of the formation of the earth’s crust; whilst the other groups were more sparsely represented in earlier geological times, but became more and more predominant until the present day.

VERTEBRATA.—

The great group of the Vertebrata comes last, with its chief characteristic of the backbone structure, which advances in complexity from the Fishes to the Higher Mammals.

A Simplified Classification of the Vegetable Kingdom.

Characters of the Plant Series.

THALLOPHYTA.—

The first series, the Thallophytes, are simple unicellular plants, and occupy the same position in the vegetable kingdom as the Protozoa do in the animal kingdom. Fossil remains of these organisms seem to be fairly well distributed throughout the entire geological series, but, owing to the soft structure of the fronds in most of the types, it is often a matter of doubt whether we are dealing with a true thallophyte or not. Many of the so-called sea-weeds (fucoids) may be only trails or markings left by other organisms, as shell-fish and crustaceans.

BRYOPHYTA.—

The second series, the Bryophytes or moss plants, are represented in the fossil state by a few unimportant examples.

PTERIDOPHYTA.—

The third series, the Pteridophytes, includes the Ferns found from the Devonian up to the present day, Horse-tails and allied forms, like Equisetites, and the Club-mosses and Lepidodendron of the Carboniferous period in various parts of the world.

PTERIDOSPERMEAE.—

The fourth series, the Pteridospermeae, comprises some of the earliest seed-bearing plants, as Alethopteris and Neuropteris. They occur in rocks of Upper Palaeozoic age as far as known.

GYMNOSPERMEAE.

The fifth series, the Gymnospermeae, contains the most important types of plants found fossil, especially those of the primary and secondary rocks: they were more abundant, with the exception of the Coniferae, in the earlier than in the more recent geological periods.

ANGIOSPERMEAE.—

The sixth series, the Angiospermeae, comprises all the Flowering Trees and Plants forming the bulk of the flora now living, and is divided into the kinds having single or double seed-leaves (Monocotyledones the Dicotyledones respectively). This important group came into existence towards the close of the Cretaceous period simultaneously with the higher mammals, and increased in abundance until modern times.