274 PROCEEDINGS OP SOCIETIES. 



it appeared, that in proportion as we rise from the ancient to the more modem 

 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 



