174 THE LIAS AMMONITES. 



short conical processes, and the two superior or dorsal pair are conjoined and dilated 

 into a muscular 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 labial 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 opJithalmic 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 Aphysia. The eyes are pedunculated, 

 and of a simple structure : the organ of hearing was not detected. 



The branchiae, four in number (fig. 25 h, b), 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 c?). 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 sho\vn 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 published, in the 

 ' Philosophical Magazine,'^ a paper on the action of the siphuncle in the Pearly Nautilus, 

 in which I pointed out what appeared to me, 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 fluid, the place of which is alternately changed from the pericar- 

 dium to the siphuncle, we shall find in this shifting fluid an hydraulic 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 fllled 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. "^ 



1 ' The Londou and Edinburgh Philosophical Magazine,' vol. xii, p. 503, 1838. 



2 Bridgwater Treatise, 'Geology and Mineralogy,' vol. i, p. 325, 1836. 



