The classification of the Foraminifera has proved a matter of considerable difficulty. The older arrangements were unnatural, as being based wholly on the form of the shell, a point in which the Foraminifera show a most marvellous variability. For this reason the artificial systems proposed by D'Orbigny and Max Schultze have now been generally abandoned, and their place has been taken by the schemes of classification put forward independently and almost simultaneously by Professor Von Reuss upon the Continent, and by Dr Carpenter, Mr Parker, and Professor T. Rupert Jones in this country. Both these arrangements agree in the essential feature that they divide the Foraminifera into two great primary divisions, in accordance with the nature of the shelly investment. In the one division (Imperforata), the test is not perforated by pseudopodial apertures, and it may be either "arenaceous" or "porcellanous." In the other division the test is perforated by more or less numerous pseudopodial foramina, and to this division the name of Perforata is applied. The following tables exhibit the arrangements proposed by Carpenter, Parker, and Rupert Jones, on the one hand, and Reuss, on the other hand; the former being the most natural, and the one most widely adopted:

Classification of the Foraminifera, according to Carpenter, Parker, and Rupert Jones.

Sub-Order I. Imperforata. - Test membranous, calcareous, or arenaceous, not perforated by pseudopodial foramina.

Family 1. Gromida. " 2. Miliolida. " 3. Lituolida.

Sub-Order II. Perforata. - Test perforated by pseudopodial foramina, generally calcareous.

Family 1. Lagenida. " 2. Globigerinida. " 3. Niimmulinida.

Classification of the Foraminifera according to Reuss. I. Foraminifera with a Non-perforate Test.

A. With arenaceous tests.

1. Lituolidea.

2. Uvellidea.

B. With compact, porcellanous, calcareous tests.

1. Squamidinidea.

2. Miliolidea.

3. Peneroplidea.

4. Orbitulitidea.

II. Foraminifera with a Perforate Test.

A. With glassy, finely porous, calcareous tests.

1. Spirillinidea.

2. Ovulitidea.

3. Rhabdoidea.

4. Cristellaridea.

5. Polymorphinidea.

6. Cryptostegia.

7. Textilaridea.

8. Cassidulinidea.

B. With an exceedingly porous, calcareous test.

1. Rotalidea.

C. With a calcareous shell, traversed by a ramified canal-system.

1. Polystomellidea.

2. Nummulitidea.

With regard to the classification of the Foraminifera, the author may be excused for quoting some remarks on this subject made by Mr Henry Bowman Brady, F. R.S., one of the highest living authorities on this group of organisms ; since they not only have a most important bearing upon the special point in question, but forcibly express the principles which should guide the philosophic naturalist in his systematic treatment of all such variable forms of life:* "A purely artificial classification is ill adapted to the conditions presented by a class of organisms like the Foraminifera, largely made up of groups of which the modifications run in parallel lines. This 'isomorphism,' demonstrated chiefly by the labours of Messrs Parker and Jones; whilst it is the source of most of the difficulties the systematist has to contend with, is, at the same time, the key to the natural history of the order. It exists not merely between a single series, in one of the larger divisions, and a single series in another, but often amongst several series even of the same family. It not unfrequently happens that a member of one group presents a greater similarity to its isomorph in another group with which it has no relationship, than it does to any other member of its own group. Take a familiar illustration - suppose the fingers of the two hands to represent the modifications ('species') of two such parallel types of Foraminifera: the thumb of one hand resembles more closely the thumb of the other hand than it does any other of the fingers on its own. In other words, the extreme member of one series resembles more closely its isomorph in the other series than it does its own nearer relations, and so on through the remaining members of the respective groups. Under conditions like these, artificial subdivision, based upon minor morphological characters, is certain to infringe the order of nature. Its tendency is to separate forms closely allied, and in many cases to place together such as have no close affinity."

* The remarks here quoted are taken from the introduction to Mr Brady's admirable 'Monograph of the Carboniferous Foraminifera of Great Britain.'