538 Forward Progression in Nautilordea and Ammonoidea. 
The mode of progression of the Ammonite animal in 
its shells must, obviously, have been similar to that just 
described for Nautilus. In the case of an Ammonite with a 
comparatively simple septal suture, or when the anterior part 
of each lobe is relatively broad, the process presents but little 
difficulty; when, however, the septal suture is very compli- 
cated, and especially when the anterior part of each lobe 
is considerably contracted, the mode of progression of the 
animal in its shell at first sight presents some difficulty. An 
easy and satisfactory explanation is, however, possible. This 
explanation presupposes the attachment of the animal to its 
shell by means of shell-muscles and an annulus like the living 
Nautilus as described in a paper to the Linnean Society 
(Trans. Linn. Soe. ser. ii. (Zoology), vol. vii. pp. 71-113). 
‘The marks left by the anterior edge of the annulus (anterior 
aponeurotic band) on the inside of the shell indicate that the 
animal progressed forward at an even rate. 
The digitations visible at the septal sutures when traced 
towards the centre of each septal surface rapidly die out and 
give place to a much more simply curved surface. During 
the formation of a septum the visceral hump of the animal 
was attached to the shell-wall, not only by the boundary of 
that part of the hump engaged in the formation of the sep- 
tum, but also by the shell-muscles and the annulus, the 
forward growth of the animal in its shell being temporarily 
suspended. On the completion of the septum and the 
renewed onwards growth of the shell-museles and annulus, 
room would be given to the animal to contract the posterior 
portion of the visceral hump inwards, and thus release and 
withdraw all the fine digitations of the mantle from the 
crumpled edge of the septum. When the creature had ad- 
vanced sufficiently far forward in its shell, further progress 
was temporarily suspended, the mantle digitations were again 
pressed outwards against the shell-wall, and the next septum 
formed, 
In some cases, when an adequate interval has not been 
attained, this results in the backward extensions (=lobes 
being pressed upon the forward extensions (=saddles) of the 
preceding septal suture, so that the resultant successive septal- 
suture lines are crowded on each other to such an extent as 
to be difficult to follow out. Such a case is well shown in 
the figure given by Pervinquiére of a specimen of Mortoni- 
ceras proratum, Coquand (Carte Geol. Tunisie.—Etudes 
Paléont. Tunisienne. I. Céphalopodes terr. Sécond. 1907, 
p. 238, fig. 97), and again (p. 298, fig. 114) in an example 
ot Acanthoceras aumalense, Coquand, ‘These cases are in all 
probability pathological. 
