FORTIFICATION OF AMMONITES. 339 
terials, in a manner calculated to combine light- 
ness and buoyancy with strength. 
First, The entire shell, (PL 35,) is one con- 
tinuous aixJi, coiled spirally around itself in such 
a manner, that the base of the outer whorls rests 
upon the crown of the inner whorls, and thus 
the keel or back is calculated to resist pressure, 
in the same manner as the shell of a common 
hen's egg resists great force, if applied in the 
direction of its longitudinal diameter. 
Secondly, besides this general arch-like form, 
the shell is further strengthened by the insertion 
of ribs, or transverse arches, which give to many 
of the species their most characteristic feature, 
and produce in all, that peculiar beauty which 
invariably accompanies the symmetrical repeti- 
tion of a series of spiral curves. (See PI. 37, 
Figs. 1—10.) 
From the disposition of these ribs over the 
occupied by the body of the animal after it had shrunk within 
its shell, at the moment of its death, leaving void the outer por- 
tion only of its chamber, from a. to b., to receive the muddy 
sediment in which the shell was imbedded. 
I have many specimens from the lias of Whitby, of the Am- 
monites Communis, in which the outer chamber thus filled with 
spar, occupies nearly the entire last whorl of the shell, its largest 
extremity only being filled with lias. From specimens of this 
kind we also learn, that the animal inhabiting the shell of an 
Ammonite, had no ink bag ; if such an organ existed, traces 
of its colour must have been found within the cavity which con- 
tained the body of the animal at the moment of its death. The 
protection of a shell seems to have rendered the presence of an 
ink bag superfluous. 
