110 
NATITILOIDEA. 
Fig. C represents the base of the body-chamber of a larger 
specimen than that from which A and B were drawn, and is de- 
signed to show more distinctly the pitted and rugose surface of the 
cast, proving, as already remarked, the strong attachment of the 
muscles at the angles of the whorl. Part of the test {t) has been 
removed in order to expose this part of the muscular impression 
more completely. 
On examining the interior of the body-chamber of the shell of 
the recent Nautilus (either N. pompilius or X. umhilicatus) two 
somewhat inconspicuous lines (fig. E, m) are observed, enclosing a 
space which on the dorsal and ventral sides of the shell forms a 
narrow band, — the impression of the annulus, — but expands at each 
side into an irregularly oval space, — the impression of the shell- 
muscle, — of which the outer boundary is strongly arched forwards. 
Corresponding in direction with the line forming the outer boundary, 
and covering the whole of the space between this and the last- 
formed septum, are a series of very fine impressed lines (marked I 
in the fig.). These lines indicate successive points of attachment of 
the upper edge of the shell-muscle, representing a gradual forward 
movement, little by little, of the auimal in its shell during growth. 
It should be observed that the irregular thick line constituting the 
lower boundary of the muscular impression is only seen where the 
muscle was last attached. 
That the organic attachment of the shell-muscles of the recent 
Xautilus to the shell was very slight (thus contrasting strongly with 
Coelonaut lilts) has been pointed out by J. D. Macdonald’ and sub- 
sequent writers". Macdonald describes very graphically what he 
conceives to be the mode of action of these “ great lateral muscles ” 
as he terms them : — 
“As though preparatory to the complete separation of the body 
of the Cephalopod from the shell, which is usually present in the 
lower genera, the fasciculi composing the lateral muscles in Nautilus 
do not perforate the mantle, and therefore cannot be directly fixed 
into the shell ; they are, however, connected with it through the 
medium of thin filmy layers of a corneous texture, which frequently 
remain attached to the shell after the animal has been removed. 
The feeble hold of those muscles, even in a very recent state, is 
thus readily accounted for. Indeed, it is highly probable that the 
^ Proc. Koy. Soc. 1856-7, vol. viii. p. 381. 
- Prof. Blake, Brit. Foss. Ceph. pt. i. p. 10 ; see also “ Note on the Pearly 
Nautilus'' by E. A. Smith, F.Z.S., in ‘ Journ. of Conchology ’ for Oct. 1887 
also ‘ Catalogue Fossil Cephalopoda,’ British Museum (Nat. Hist.), pt. i. 1888, 
p. xi. 
