46 



slip from their attachments, and form new ones at a determinate distance from 

 the former ; the operation being analogous, remotely indeed, to a partial moulting 

 or casting off an exuvial covering, the loss of which is repaired by the secretion 

 of the partition closing the deserted chamber, and by the successive additions 

 to the margins of the chamber of occupation. This process becoming repeatedly 

 necessary from the increasing bulk of the animal, is the obvious physical cause 

 of the camerated portion of the shell ; to have filled up the whole of the cham- 

 bers with calcareous matter during a more gradual process of advancement, 

 would have produced an incumbrance incompatible with the locomotive faculties 

 of the inhabitant. 



One important point in the history of the Pearly Nautilus, viz. the exact relation 

 of the posterior membranous tube to the siphuncular apeitures of the septa and 

 intermediate chambers, can receive but little elucidation from the present in- 

 qmry, in consequence of the loss of the shell from which the specimen was taken. 

 In sections of recent shells its dried remains may occasionally be seen of a black 

 colour and pergameneous texture, continuing from septum to septum as far as 

 the central or first-formed chamber : and a further confirmation that tliis is the 

 true structure of the parts, is afforded by the fossil shells of this genus. In some 

 polished sections of these remains, not only is the continuation of the tube 

 through all the chambers evident, but it is seen to become slightly dilated in 

 them, and in some instances appears also to have been reflected over the outer 

 part of the testaceous tube prior to being continued across the chamber to the 

 next partition*. There is no indication, however, of the latter structure in the 

 recent shells, where the membranous tube is preserved ; but there is a delicate 

 pellicle, distinct from the tube, continued over the outer part of the testaceous 

 tube, and also over the whole inner surface of the chamber. 



The above appearances in the fossil shells have been deemed confinnatory of 

 the In-pothesis of Dr. Hookef, who supposed Nautilus Pompilius to have the 

 power of generating air into, and expelhng it from the deserted chambers ; and 

 that it regulated, in the same manner as Fish by means of their air-bladders, its 

 ascent and descent in the water. Mr. Parkinson, in adopting this theory, as- 



* Parkinson's Organic Remains, vol. iii. p. 102. pi. vii. These fossUs are now preserved in the 

 Museum of the Royal CoUege of Surgeons, 

 t Pliilosophical Experiments and Observations : 8vo, Lond. 1726. pp. 307. 310. 



