GENUS ORBICULINA : — ORGANIZATION. 
551 
them : — First, they so closely resemble one another, as to be nndistingnishable, in their 
early condition. Second, they correspond in every particular, so far as regards the 
structure ol their minute parts. Third, the assumption of the cyclical plan of growth 
does not take-place at any one fixed epoch of development, but may occur at various 
periods. Fourth, the per^istence of the original plan of growth througliout life, can- 
not be fairly regarded as anything else than an arrest of development, such as we 
shall presently see to be a common occurrence in Orhiculinu, as in OrhitoUtes, in 
regard to other particulars 90). 
88. Turning now from the general plan of growth to the minute structure of the 
individual parts of OrhicuUna, we continue to find a very close conformity to the type 
of OrhitoUtes. The texture of the shell is precisely the same; and it exhibits no 
other peculiarity than a minute punctation of the superficial layers (Plate XXVIII. 
fig. 15), which at first suggests the idea of apertures*, but which is found on careful 
exatnination of transparent sections (Plate XXIX.fig. 2) to be due to a mere thinning 
of the shell at certain points, so as to give an appearance of cellular areolat ion closely 
resembling what is seen in OrhitoUtes (Plate VI. fig. 5). 
89. Not having had the good fortune to obtain specimens in which the animal 
body has been preserved, I cannot speak as confidently on the subject of its conforma- 
tion, as I could in regard to that of OrhitoUtes but a comparison of the features 
which present themselves in the structure of the calcareous skeleton of the two types, 
can leave no reasonable doubt that the general arrangement of its segments of 
sarcode must have been precisely the same. For the conformation of the chambers 
and passages, as displayed by a horizontal section (Plate XXIX. fig. 1), shows that 
the soft body must consist of a succession of bands of sarcode, each band swelling at 
intervals into larger segments; that the segments of each band usually alternate with 
those of the bands internal and external to it, so as to be opposite to the interv’^als 
between them ; and that the stolons which connect one band with another pass-forth 
from pores in the intervals between the segments of one, into the segments of the next, 
those of the outermost band emerging from the margin as pseudopodia. By the 
coalescence of these a new band would originate, which would become thickened 
into segments opposite the pores of the preceding, and would give olf its own pseudo- 
podia from the intervals between the segments. 
90. Again, by an examination of the natural margins of Orhicidinoe (Plate XXVIII. 
figs. 6, 7? 18? 19), and by a comparison of these with vertical sections (Plate XXIX. 
fig. 3), it becomes evident that the same variety exists as in OrhitoUtes, in regard to 
the increase of the disk in thickness by the vertical elongation of the segtnents of 
sarcode. For some speciinens are altogether conformable to the ‘simple type’ of 
OrhitoUtes, in having but a single floor of chambers (so to speak), with a single row 
of marginal pores; whilst others correspond with the complex type of OrhitoUtes, in 
* Professor Williamson {loc. cit.) has described these punctations as perforations for the passage of pseu- 
dopodia ; but I am quite certain that such is not the case. 
