GENUS CYCLOCLYPEUS: — GENERAL SUMMARY. 
561 
its resemblance to the former in external aspect and plan of growth, it is far more 
closely allied to the latter in those features of its organization which indicate its 
physiological condition. It has been shown that in Orhitolites the communication 
between the different portions of the sarcode-body is so free, that the whole may be 
regarded as a continuous mass in which the segmental division is but imperfectly 
indicated (*|[ 67); and this view is in complete harmony with the fact, that every 
addition made to the shelly disk forms (save in a few rare cases) an entire annulus. 
In Cycloclypeus, on the other hand, the chambers are so completely separated from 
each other laterally, that no other communication exists between them than such as 
may be established by the interseptal canals; and the communications between the 
chambers forming successive annuli, are only large enough to allow the passage of 
narrow bands of sarcode. Hence we see that there is here as much segmental inde- 
pendence as is consistent with the existence of these animals, which involves the 
maintenance of a communication between the innermost and outermost chambers, 
for the transmission of nutriment to the segments of the sarcode-body contained 
within the former. And this independence is strikingly manifested, by the frequency 
with which incomplete annuli are added to the previous margin of the disk, extend- 
ing (it may be) along not more than a third, a half, or two-thirds of the entire circum- 
ference. The want of constancy in the number and position of the communications 
between the chambers (^ 101), even in this high type of Foraminiferous structure, is 
a point of fundamental importance in the determination of the value of the shape of 
the aperture, as a character of discrimination between genera and species in this group 
of organisms. I have elsewhere shown that a like want of constancy exists in Nurn- 
mulites {op. cit. p. 24), the different septa of one and the same specimen having aper- 
tures of very varied forms. We are not in a condition to assign a positive function 
to the minute tubuli that traverse the shelly layers intervening between the chambers 
and the two surfaces of the disk. But it is quite possible that these tubuli may give 
passage to pseudopodial prolongations of extreme minuteness, which may spread 
themselves forth from the whole surface of the disk, as we know that they do from 
the larger pores of Rotalia, and may coalesce so as to form upon it a continuous layer 
of sarcode, by whose instrumentality a lamina of shell is added from time to time to 
those previously existing. That either by giving passage to threads of sarcode, or 
by conveying organizable fluid, they furnish the means for progressive increase of 
the shell in thickness, would seem a very probable account of their use. Whether 
this hypothesis, however, be correct or not, there can be no reasonable doubt that 
the minute organization of these shelly layers in Cycloclypeus and Numtnulites, — an 
organization as high as that of dentine, — is a feature of high elevation, as compared 
with the simple concretionary condition of the calcareous skeleton in the three genera 
previously examined. 
110. Thus, then, in the almost complete isolation of the segments, in the enclosure 
of each of them in its own proper wall, in the interposition of an intermediate skele- 
