138 
The part of the Cornu ammonis which next particularly demands 
our attention is the siphuncle ; and which has hitherto been so rarely 
made out, as to have led many to doubt whether it existed or not. 
Its existence being, however, proved in several specimens, analogy 
leads us to conclude that it really exists in all ; and that it served the 
same purposes as the siphuncle in the Nautilus, the enabling of the 
animal to regulate its ascent and descent in the water. 
The spondylolite, Plate IX. Fig. 3, strongly proves the existence of 
the siphuncle, by the deeply-sunk circular cavity which exists on the 
outer side of this fossil body, where, doubtlessly, the siphuncle had 
passed. In the very rare specimen, Plate IX. Fig. 4, which formed 
a part of the Leverian collection, the siphuncle itself is seen, in situ. 
The situation of the siphuncle is not always the same, in every spe- 
cies : in some, it runs along the back of the shell, as in the specimen 
Fig. 3 ; in others, it runs along the inner part of the whirl, as in the 
specimen represented Fig. 4 ; and, unless I am very much deceived, 
a siphuncle runs along both the outer and inner side of the whirls of 
the specimen, Fig. 5. 
The exact figure of the siphunculus is not, I believe, yet known. I 
have destroyed many specimens, with the hope of obtaining informa- 
tion in this respect ; but without decided success. In general it ap- 
pears to be formed by a uniform cylindrical tube, regularly increasing 
in size as it proceeds from the central to the outer whirls. I think, 
however, that in some parts of the specimen, Plate IX. Fig. 5, the 
siphuncle seems to be a little contracted in those parts where the 
septa unite with the outer shell ; but that, in other parts, the septa 
appear to be formed of two shelly plates, which, separating as they 
approach the outer shell, seem to admit a duplicature of the siphuncle 
to dip in between them. Some of this appearance may however be a 
deceptio visus, proceeding in some measure from the direction in 
which the parts are divided. 
Bertrand, Diction, des Fossiles, p. 175, and others, who have been 
