MM. Foord & Orick—Shell-muscles of Ceelonautilus, ete. 497 
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 J in Fig. Z). 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 animal in its shell during growth. It should be observed that 
the 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 
Nautilus to the shell was very slight (thus contrasting strongly 
with Celonautilus) has been pointed out by J. D. Macdonald? and 
subsequent writers.? Macdonald’s description is so important and 
interesting in this connection that we quote it 7m extenso :— 
“With reference to the action of the great lateral muscles of 
Nautilus, the following ideas have suggested themselves to my mind, 
“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 
fixity of the body of Nautilus during the inhalation and forcible 
ejection of the respiratory currents is effected by the shell-muscles 
reacting upon one another, on the principle of a spring purchase, 
rather than by simple traction, as illustrated by the withdrawal of 
a Gasteropod within its retreat, or the closure of a conchifer by the 
adductor muscles. 
“This view, which is supported by the foregoing facts, has its 
principal basis in the line of direction of the shell-muscles, and the 
angle at which they meet one another, at the root of the funnel- 
lobe; for, the outer extremity of each being fixed, it follows that 
the first effect of the contraction of the muscular fibres would be to 
increase the angle just noticed; and this cannot possibly be accom- 
plished, according to the recognized laws of muscular action, with- 
out tending to throw apart the points of origin, or, in other words, 
exerting outward pressure against the internal wall of the shell, 
and thus, as it were, jamming the occupant tightly in its cell.” 
In order that the above description may be more readily compre- 
hended we here append a reduced copy of the figure given by Sir 
Richard Owen in his ‘“ Memoir on the Pearly Nautilus,’ 1832, 
plate 3, fig. 2. 
A comparison of the muscular impressions of Cclonautilus with 
those of the recent Nautilus points to the conclusion that the animal 
1 Proc. Roy. 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.. m ‘‘ Journ. of Conchology ” for Oct. 1887 ; 
also ‘ Catalogue Fossil Cephalopoda,’ British Museum (Nat. Hist.), pt. i. 1888, p. xi. 
DECADE I1I.— VOL. VI.—wNO. XI, 32 
