ON THE STRUCTURE OF CAMERATED SHELLS. 
119 
The formation of the septa is undoubtedly due to the constant 
onward growth of the shell, season by season, and in the female to 
the periodic development of the ova within the ovary of the 
parent ; producing when discharged from the shell, a correspond- 
ing reduction in the size of the soft parts of the animal, and 
necessitating an equal reduction in the space of the body- 
chamber. 
In youth these septa represent periods of rest in shell-growth ; 
in middle-life periods of fertility ; in age reduction of the shell 
to suit the reduced size of the Mollusk. 
In this respect the septa in Nautilus agree with those found 
in other Mollusca. 
“ The line of attachment of both the muscles and the cincture 
progressively advances with the. growth of the animals. A 
certain portion of the fundus of the shell thus becomes vacated, 
and the Nautilus commences the formation of a new plate for 
the support of the part of the body which has been withdrawn 
from the vacated shell. The formation of the plate proceeds 
from the circumference to the centre, and there meeting the 
conical process of the mantle, which retains its primitive attach- 
ment, the calcification is continued backwards for a short dis- 
tance around the process which now forms the commencement 
of the membranous siphon, and acquires the partial protection 
of the calcareous tube. An air-tight chamber is thus formed, 
traversed by the siphon, which perforates its anterior wall or 
septum ; by a repetition of the same process a second chamber 
is formed, included within two perforated septa ; and similar 
but wider partitions continue to be added, concurrently with the 
formation of the new layers which extend and expand the mouth 
of the shell, until the animal acquires its full growth, which is 
indicated by the body having receded for a less distance from 
the penultimate septum before the formation of the last septum 
is begun.” (Owen, op. cit. p. 592.) 
In Aturia zic-zac (figs. 3, 4), the siphuncle is not con- 
tinuous, but is composed of a series of funnel-shaped tubes 
inserted into each other. 
In Ostrea I have observed that the shell-muscle appears to be 
the last point to be elevated upon the new shell-layer or septum 
formed within the lower valve, so that a section of an aged 
oyster exhibits a series of incipient siphuncular depressions, 
each fitting into that preceding it. (Plate LXXXII. fig. 12.) 
The attachment of the muscle in the oysters may perhaps offer 
the true explanation of the mode of the formation of the shelly 
tube or siphuncle in the Pearly Nautilus. 
If we will only bear in mind this fact, that the animal of the 
Oyster and the Nautilus are each alike compelled by the constant, 
though almost imperceptible, growth of the mouth or border of 
