GENUS ORBICULINA : — ORGANIZATION. 
549 
Other types, we shall be in a position to inquire what value is really to be attached 
to such a character as a basis for classification. 
83. Organization. — Among- the diversified forms that are presented by the species 
before us, it is of course necessary to select some particular type as that with which 
others may be compared; and this I consider to be the form delineated in Plate XXVIII. 
fig. 5 ; since by far the larger proportion of the very numerous specimens I have 
examined show such a degree of approximation to it, that their differences may fairly 
be set-down to the account of incomplete development. The general aspect of a 
typical Orhiculina, then, differs but little from that of OrbitoUtes, except in the pro- 
minence of its nucleus, and in the peculiar spiral disposition of the bands by which 
the surface of the nucleus is marked. The species never attains, so far as I am 
aware, the dimensions of OrbitoUtes ; the diameter of the largest recent disk in my 
possession (one of Mr. Cuming’s Philippine specimens) being ‘20 of an inch ; whilst 
of the specimens which I have seen from the West Indian and ^gean seas, none 
exceed -12 of an inch. Mr. Carter*, however, describes under the name of Orbito- 
lites Malabaricus a fossil OrbicuUna 90), whose specific identity with the type 
before us I see no reason to doubt, as attaining a diameter of from 7 to 8 lines. The 
thickness of the disk is usually less in proportion to its diameter, than it is in Orbi- 
toUtes; but this is by no means a constant difference, depending as it does merely on 
the relative increase of the columns of sarcode in the vertical direction, as compared 
with the extension of the surface by the addition of new annular bands. 
84. On more closely comparing the marginal portion of such a disk with the cor- 
responding portion of one of those forms of OrbitoUtes, in which the concentric annu- 
lation is strongly marked externally whilst the transverse divisions of each annulus 
are scarcely indicated (5[ 49), no essential difference is perceptible between them; 
and although these transverse divisions are usually but very faintly indicated in 
OrbicuUna (Plate XXVIII. figs. 13, 14), yet they are sometimes very obvious, as is 
seen in figs. 15, 16. In the fossil OrbicuUna just adverted to, the surface-markings 
(figs. 21, 22) are entire!}^ conformable to the ordinary type of OrbitoUtes. The 
margin itself exhibits one or more ranges of pores (figs. 6, 7 , 18, 19) arranged after 
precisely the same fashion as those of OrbitoUtes: these ranges are more numerous 
in the fossil than in the recent forms of this type. 
85. The internal structure of OrbicuUna presents such a general conformity to that 
of OrbitoUtes, that it will not be requisite here to do more than specify the points of 
agreement and of difference. The general aspect of the horizontal section of such a 
typical specimen as is shown in Plate XXVIII. fig. 1 1, does not differ from that of a 
corresponding section of OrbitoUtes in any other essential particular than the dispo- 
sition of the central portion, in which the successive additions to the first-formed 
chambers are so made as to produce a spiral, instead of a concentric disk. When we 
examine this central portion more closely, we see that its plan of growth is exactly the 
* Annals of Natural History, New Series, vol. xi. p. 425. 
4 c 2 
