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possesses all the attributes of the body of which it is an extension, and can maintain its existence with equal readiness, either in a separate state or in continuity with the stock of which it is an offset. Although, therefore, there are certain types of Foraminifera in which such offsets appear invariably to separate themselves before the consolidation of the shell, so that the original body never adds to the number of its segments, and the shell remains "monothalamous,"—whilst there are others in which they ordinarily remain in connection with the original stock, so as progressively to augment the number of the segments and of the chambers of the "polythalamous" shell, often to an indefinite extent,—I cannot see any such difference between the physiological conditions of the newly-formed segment in the two cases, as would be required to justify the erection of the Monothalamia into a distinct order. Moreover, we find that each of these groups, as ordinarily constituted, contains forms which in principle should rank with the other. Thus the continuous spiral shells which are known as Spirillinæ or Cornuspiræ, having their cavities undivided by septa, are always ranked among Monothalamia; but as they have the capacity for indefinite extension, which is characteristic of the Polythalamia, they need nothing but segmental division to turn them into Rotaliæ or Spiroloculinæ. Hence, such shells though actually monothalamous, are potentially polythalamous; and to rank them with Gromiæ, Lagenæ, or Orbulinæ, whose increase can only be effected by the complete detachments of the superfluous segments of sarcode, and by the formation of new and independent envelopes for these,—the enlargement of their shells being forbidden by their shape, would be antagonistic to the very principle on which the differentiation is based. I have recently been investigating another type, not until lately ranked among Foraminifera, which presents a condition of precisely the converse nature. In Dactylopora and Acicularia (as I shall more fully explain in my forthcoming Monograph), we have composite organisms of definite form made up by the aggregation of chambers which have no internal communication with each other, each being as distinct from the rest as the chambers of a heap of Lagenæ, and being only united by external adhesion. Such organisms, therefore, although actually polythalamous, are essentially monothalamous; since the sarcode-body, contained within each chamber, is as independent of the bodies enclosed in the neighbouring chambers, as it would have been if these chambers had been altogether disconnected. Again, there are certain Polythalamia, the successive chambers of whose shells, although formed by continuous gemmation the one from the other, are so slightly connected as to be easily separable by accidental violence, and of which the animals can maintain their lives just as well when they are thus broken up into distinct segments as when retaining their original continuity; such, again, may be regarded as potentially Monothalamous; and the fact that the segments of sarcode, as they were successively budded off from the stock, formed their shelly investments before,