l xx . PRESIDENT'S ADDRESS. 



of the cone, produces a complete spiral shell, such as that 

 of the Planorbis, in which all the convolutions are on the same 

 plane. The substance of univalve shells generally contains less 

 animal matter than the Bivalves, and it is frequently found 

 porcellaneous. Several of the Gasteropods have an accurately 

 fitted cover (operculum} to the mouth, and so attached to the body 

 that when withdrawn it is completely shut in. Only two British 

 land-shells have an operculum ; some construct a temporary 

 one by a secretion, which hardens, and includes a bubble 

 of air. Many Gasteropods are either naked or shell-less ; 

 this is the case with the Slugs. The Testacella has a very 

 small shell upon its tail. There is one marine Order, the 

 Nudibranchiata, whose respiratory apparatus extends over so 

 large a part of the external surface, that the investment of 

 the body in a shell would obstruct its functions. The most 

 remarkable departure from the general type of the shell's 

 conformation is the Chiton ; this animal is more closely allied 

 to the Limpet than to any other Gasteropod, while the shell, 

 instead of being cone-shaped, is composed of a number of pieces 

 jointed to each other. 



The highest group of Molluscs represented in the seas of 

 the present day is the Cephalopods, approximating to a certain 

 degree to the Vertebrates ; at the same time the lowest are only 

 slightly removed from the Gasteropods. The feet, which are 

 disposed in a radiating form around the mouth, must be 

 regarded as highly developed tentacula, and have nothing in 

 common with the locomotive organs of other animals. Some 

 have shelly internal supports, and one genus, the Argonaut, 

 or Paper Nautilus, has an external protective shell. The 

 Nautiloids appear suddenly in the Lower Silurians ; from the 

 Devonian period they gradually diminish, and in the Mesozoic 

 they are considerably predominated by the Ammonites. These 

 are remarkable for the ornate markings on the surfaces of their 

 shells, and for the waved edges of the partitions, which, besides 

 giving a support to each section, contributed greatly by the union 

 of lightness and strength to the buoyancy of the shell. These 



