PROCEEDINGS OF SOCIETIES. 275 



descend into the water. That this is erroneous must be evident to any one who 

 reflects that this theory allows only of a change of place in the balancing fluid, 

 consequently the aggregate weight of the animal and its shell remains the same. 

 The error into which the Doctor has fallen arises from his having overlooked 

 the fact that the pericardial bag communicates before by two openings with the 

 branchial cavity, into which the sea-water is constantly flowing to bathe the 

 gills; whilst posteriorly there proceeds from the sac a membranous tube that 

 passes through all the air-chambers of the shell. Thus, when the animal is 

 creeping upon its fleshy foot at the bottom of the sea, the water, admitted by the 

 openings just alluded to, distends the bag, and then flows through the entire 

 tube ; it is clear, therefore, that the weight of the sea-water contained in the bag 

 and siphuncle is the amount of ballast necessary to retain the animal at the bot- 

 tom, by rendering it specifically heavier than the ambient fluid ; but supposing 

 the Nautilus desires to rise to the surface, all that is required to render it spe- 

 cifically lighter is to get rid of its watery ballast. This it accomplishes by 

 contracting the bag, and ejecting the fluid through the openings that communicate 

 with the branchial cavity ; a partial vacuum being thus produced in the bag, — 

 that portion of the water which had distended the pipe now flows into it, and a 

 second contraction is all that is requisite in order to render the animal and its 

 shell specifically lighter ; and thus it is enabled to rise to the surface. When it 

 wishes to descend it admits the sea-water, bscomes heavier, and gradually sinks 

 to the bottom. 



The concluding lecture of Mr. Wright's course on Fossil Organic Remains was 

 delivered on March 6, to as numerous an audience as had attended either of the 

 four preceding lectures. 



Mr. Wright demonstrated the organization of the Nautilacece, and described 

 the mechanism by which these syphoniferous mollusks are enabled to ascend and 

 descend in the water. From the consideration of the living representative of 

 this once-extensive group, he proceeded to describe the fossil Nautili of our 

 neighbourhood, each system of rocks having forms peculiar to them. The Ortho- 

 ceratite and Lituite were shown to be genera of this family, that commenced and 

 terminated their career in the waters that deposited the transition series. The 

 Ammonitidm are an extensive fossil family, composed of five genera, each of which 

 have received names from their supposed resemblance to particular objects. The 

 Ammonites have the shell-discoidal more or less compressed, and the spine rolled 

 upon the same plane ; the partitions terminate in flower-like ramifications upon 

 the surface, and the last chamber is very capacious, and lodged the body of 

 the animal ; the surface of the shell is ornamented with an immense variety of 

 forms, as ribs, flutings, tubercles, &c, which are disposed with exquisite sym- 

 metry over the entire shell. These are so many mechanical contrivances fof 



