PROCEEDINGS OP SOCIETIES. 
.275 
descend into the water. That this is erroneous mtkt\ 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, becomes 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 
Ammonitidce 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 for 
