119. In my preceding memoirs, I have shown that two very dissimilar types of structure present themselves among the Foraminifera; one characterized by its simplicity, the other by its complexity. In the former, of which
Orbitolites
,
Orbiculina
, and
Alveolina
are typical examples, the calcareous skeleton does not present any definite indications of organization, but seems to have been formed by the simple calcification of a portion of the homogeneous sarcode-body of the animal; that the sarcode-body is but very imperfectly divided into segments, the communications between the cavities occupied by these segments being very free and irregular; the form of the segments themselves, and the mode of their connexion, are alike inconsistent; and even the plan of growth, on which the character of the organism as a whole depends, though preserving a general uniformity, is by no means invariable maintained. In the latter, to which
Cycloclypeus
and
Heterostegina
belong, we find the calcareous skeleton presenting a very definite and elaborate organization; the several segments of the body are so completely separated from each other, that they remain connected only by delicate threads of sarcode; each segment thus isolated has its own proper calcareous envelope, which seems to be moulded (as it were) upon it, and this envelope or shell is perforated with minute parallel tubuli, closely resembling those of dentine except in the absence of bifurcation or ramification; the partition-walls between adjacent segments are consequently double, and are strengthened by an intermediate calcareous deposit, which is traversed by a system of inosculating passages that seems properly to belong to it. The form of the segments, their mode of communication, and consequently the general plan of growth, have a very considerable degree of constancy; and altogether the tendency is strongly manifested in this type, to the greater individualization of the parts of the composite body, which in the preceding must be looked upon rather as constituting one aggregate whole. 120. I purpose in the present memoir to carry on this contrast, by presenting a detailed comparison of the structure of two generic forms, which, whilst they so far agree in general plan of growth as not only to have been ranked by M. d'Orbigny side bv side in his order
Hélicostègues
, but to have been placed by other systematists in close apposition, differ in the most marked manner as to all the particulars just enumerated. Both these types are of peculiar interest:- the first,
Peneroplis
, on account of the very wide range of variation it presents, which has led to the establishment of three
genera
, apparently distinguishable by well-marked differences in conformation, upon what I feel satisfied will prove to be but individual modifications of one and the same
specific type
;— the second,
Operculina
, as being the nearest existing representative of
Nummulites
, and consequently as affording not merely the key to the elucidation of the structure, but also the basis for the determination of the value of the reputed species, of that genus, by the study of the range of variation which it presents; this range being, though more restricted than in the preceding case, still quite sufficient to justify a large multiplication of species, in the estimation of those who do not practise that extended method of comparative inquiry, on the importance of which I have dwelt in a former Memoir (74). With the latter of these genera, as also with
Nummulites
, I shall prove that the genus
Amphistegina
is closely allied; although M. d’Orbigny, misled by the marked want of symmetry and by the alternation in the disposition of the chambers, which are exhibited by certain forms of that type, has placed it in a different order,
Entomostègues
. For I shall have to show that a gradational variety in this respect, ending in complete symmetry, may coincide with such a uniformity in general structure, that even a very decided departure from symmetry must be regarded as a character of little value in classification, compared with agreement in the organization of the shell and in those peculiarities in the conformation of the animal which are indicated by it; and further, that a most marked difference in degree of organization exists between two species of
Amphistegina
, which so closely resemble each other externally that the young of one may easily be mistaken for the adult of the other.
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