274 
PROCEEDINGS OF SOCIETIES. 
it appeared, that in proportion as we rise from the ancient to the more modern 
systems of strata, the ichthyoid reliquiae approximate in their form and dermal 
covering the fishes of the present epoch. The change which appears to have taken 
place in the form of the tail of fishes after the deposition of the magnesian lime¬ 
stone was particularly dwelt upon; namely, that in those found in the Silurian 
and carboniferous strata the tail was unequally bilobed, whereas those entombed 
in the upper formations of the saliferous, oolitic, and cretaceous systems have 
the tail equally bilobed. As the large Lizard-like fishes were the dominant class' 
in the seas anterior to the creation of the reptiles, they attained enormous 
magnitude during the carboniferous era; into the waters which had deposited 
the magnesian limestone, however, reptiles were for the first time introduced, 
and these took the place of their ichthyoid predecessors. The lecturer described 
some of the striking forms which characterize the different systems of strata, and 
showed that important changes had taken place in the family of Sharks, certain 
sub-families being confined to particular formations; and how that the present 
forms of that voracious family, with lancet-shaped teeth, had been created about 
the period that the last of the long line of marine reptiles had become extinct. 
In entering upon the consideration of Fossil Invertebrata, Mr. W. observed that 
it would be impossible to describe, as minutely as he wished, the organization of 
this highly-interesting division of his subject, seeing that his course was limited; 
he would therefore select those fossils which the student is most likely to encounter 
in his rambles. The lecturer gave an outline of the organization and classifi¬ 
cation of molluscous animals, and described the most important traits in the 
structure of the Cephalopodes , or those which have their arms or tentacules 
disposed around the head, like the Cuttle-fish. Before describing the fossil genera 
of this class, he observed that it would be desirable to impart a correct notion of 
the peculiar structure of the living representative of the testaceous group of this 
class, which in byepast ages had fabricated and inhabited those singularly 
abundant fossil shells, called Ammonites, Baculites, Scaphites, &c. The Nauti - 
lacece , he said, were once an extensive family, and the last descendant of this 
line was the constructor of that elegant shell called the Pearly Nautilus, now on 
the table. This shell is a conical tube, which is coiled upon itself; internally 
it is divided into a series of air-chambers by transverse partitions. Through the 
centre of each division is a small hole, which in the recent state communicates 
with a pipe that runs through all the chambers. This pipe terminates in a sac 
or reservoir, which surrounds the heart of the creature. Dr. Buckland, in his 
late treatise, founds a theory upon the action of this pipe or siphuncle, which may 
be thus briefly stated. He supposes that the undulation of the fluid from the 
bag to the pipe, by compressing the air in the chambers of the shell, so alters the 
specific gravity of the animal, that by these means it is enabled to ascend or 
