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of two distinct laminæ, which diverge from each other where they give passage to the canal system, and which are often further separated by the intervention of a portion of the intermediate skeleton. The passages of communication between the chambers are so narrow, that segments of the body are much more isolated from each other than they are in the type already described; and the proper walls of the chambers seem, as it were, to be moulded upon the segments, instead of merely filling-up the interspaces, between them, as they there do. This filling up, in fact, is the office of the intermediate skeleton, which gives a solidity to the whole aggregation that it would otherwise want; and special provision, as we have seen, is made in the canal system for its nutrition. Altogether this type is the one in which the Foraminiferous structure attains its highest development, and which is most completely differentiated from every other. And the morphological variations it is known to undergo seem to me fully to justify the inference that such further variations as have been shown to occur in the Orbiculine type might be regarded as the probable source of the divergence from some common ancestral stock of the several forms whose intimate relationship I have demonstrated. The analogy of that type would suggest Heterostegina as presenting the nearest existing approximation to such a common original; and the stages of differentiation may be thus expressed:—

HETEROSTEGINE TYPE,
Diverging into
$\scriptstyle {\overbrace {\quad \quad \quad \quad \quad \quad \quad \quad \quad \quad \quad \quad \quad \quad \quad \quad \quad } }$
Orbiculina.		Heterostegina.
$\scriptstyle {\overbrace {\quad \quad \quad \quad \quad \quad \quad \quad \quad \quad \quad \quad \quad \quad \quad } }$		$\scriptstyle {\overbrace {\quad \quad \quad \quad \quad \quad \quad \quad \quad \quad \quad } }$
Amphistegina, Nummulites, Operculina.		Heterostegina, Cycoclypeus.

From my imperfect acquaintance with Fusulina, I do not feel justified in expressing its exact relationship to either of the forms included in this scheme; and, for the same reason, I abstain from connecting Orbitoides with Cycloclypeus, to which it has some features of close relationship.^[1]

After this detailed examination of the general relations of the principal modifications of two of the most strongly-marked types to be found in the whole group of Foraminifera, it seems needless for me to do more with respect to the other forms whose structure I have investigated, than to inquire how far the peculiar characters by which they are respectively distinguished show evidence of a like variability. Thus I have shown (4th series) that Calcarina is essentially distinguished from Rotalia by the extraordinary development of the intermediate or supple-

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1. The figure given by Prof. Ehrenberg, in his remarkable memoir already referred to, "Ueber den Grünsand und seine Erläuterung des organischen Lebens," Plate IV., fig. 8, and by him designated as the internal cast of Orbitoides javanicus, will be seen on comparison to present a most remarkable correspondence with figs. 10, 11, 12, of Plate XXIX., illustrating my description of Cycloclypeus.