174 
THE LIAS AMMONITES. 
short conical processes, and the two superior or dorsal pair are conjoined and dilated 
into a mnscular disk covering the whole upper part of the head ; the remaining thirty- 
eight are disposed nineteen on either side, closely overlapping one another, and all are 
directed forwards towards the mandibles. The lahial tentacles, forty-eight in number, 
extend from orifices situated on the anterior margins of four broad flattened processes 
arising opposite the base of the mandibles. The ophthalmic tentacles, four in number, 
project laterally, one before and one behind each eye ; they appear to be sensory instruments, 
and resemble the cephalic tentacles in Doris and Apliysia. The eyes are pedunculated, 
and of a simple structure : the organ of hearing was not detected. 
The branchiae, four in number (fig. 25 3, h), are inclosed in the respiratory sac, 
without branchial hearts. 
The circulating system is provided with one ventricle which is systemic, and propels 
arterial blood throughout the body (fig. 25 d). There is no ink-bag in the animals 
inhabiting these camerated shells. The funnel, which occupies the floor of the body- 
chamber, is divided by a longitudinal slit, and inside there is a valve-hke fold (fig. 25 s). 
A large dilatable and contractile tube is continued from the posterior part of the 
animal through all the partitions and chambers of the shell, as shown in fig. 24 ; this is 
the siphuncle, which is a vascular tube that opens into the cavity of the pericardium 
containing the heart, and is probably filled with sea water from the branchial (fig. 25 c) 
chamber. 
Soon after the appearance of Dr. Buckland's '"'Bridgwater Treatise," I pubhshed, in the 
'Philosophical Magazine,'^ a paper on the action of the siphuncle in the Pearly Nautilus, 
in which I pointed out what appeared to nie, as an anatomist, the discrepancy between 
the structure of the animal and the explanation given by the author, who observed, 
" The last contrivance, which I shall here notice, is that which regulates the ascent and 
descent of the animal [the Nautilus] by the mechanism of the Siphuncle. The use of this 
organ has never yet been satisfactorily made out ; even Mr. Owen's most important 
Memoir leaves its manner of operation uncertain ; but the appearances which it occa- 
sionally presents in a fossil state, supply evidence, which taken in conjunction with 
Mr. Owen's representation of its termination in a large sac surrounding the heart of the 
animal, appears sufficient to decide this long disputed question. If we suppose this sac 
to contain a pericardial fiuid, the place of which is alternately changed from the pericar- 
dium to the siphuncle, we shall find in this shifting fluid an hydrauHc balance, or 
adjusting power, causing the shell to sink when the pericardial fluid is forced into the 
siphuncle, and to become buoyant whenever this fluid returns to the pericardium. On 
this hypothesis also the chambers would be continually filled with air alone, the elas- 
ticity of which would readily admit of the alternate expansion and contraction of the 
siphuncle in the act of admitting or rejecting the pericardial fluid."- 
^ ' The London and Edinburgh Philosophical Magazine,' vol. xii, p. 503, 1838. 
2 Bridgwater Treatise, 'Geology and Mineralogy,' vol. i,p. 325, 1836. 
