PRESIDENT'S ADDRESS. Ixix, 



currents of water by the agency of the cilia for the conveyance 

 of food to the mouth. Except during the early stages of life 

 they are fixed to one spot by means of a peduncle attached to 

 one of the valves, which passes through a hole in the beak- 

 shaped prolongation of the other. The valves of these, 

 and indeed of all Bivalves are kept in place one against the 

 other, sometimes by muscles, and sometimes by two cardinal 

 teeth on the posterior border of the right valve, fitting into 

 two corresponding pits in the left ; there is also a process 

 between the two teeth for the insertion of the muscles attached 

 to the left valve ; by this means the displacement of the valves 

 is impossible and they cannot be opened entirely without frac- 

 turing the rim. Little is known of the habits of living Brachiopods, 

 as they frequent great depths, their classification having been 

 studied more by palaeontologists than by zoologists. Owing to 

 an uninterrupted duration through geological ages, and their good 

 preservation, they are better known than any other Mollusca. 



The next in order are the Lamellibranchiata, chiefly distinguished 

 by the presence of special respiratory organs, or branchiae. The 

 variety of forms in this class is considerable, and their habits and 

 modes of life are no less dissimilar. Some of them, as the Oyster, 

 are fixed by the adhesion of their valves, during the greater 

 period of their lives ; some, as the Pinna and the Mussel, are 

 attached to solid bodies by a byssus ; others, as the Pecten, 

 propel themselves by a flapping movement of their valves ; while 

 the Cockle can move by jumps over hard surfaces ; the Mya, Solen, 

 &c., bore into the sand or mud, and live in the excavations they 

 have made ; the Teredo and Pholas make their way into wood and 

 even stone. We find the Lamellibranchiata advancing in several 

 particulars towards the higher types of the Order. The next in 

 succession are the Gasteropoda. A large proportion of these 

 possess a shell within which the body can be retracted. Its 

 typical form may be considered to be a cone, with a broad base, as 

 in the Limpet. In the Pileopsis the point of the cone is prolonged 

 and somewhat turned to one side, presenting the rudiment of a 

 convolution. The increase of tendency to deflexion in the axis 



