6 



seven in number in an average adult specimen. The last 

 turn of the shell is known as the body whorl and is 



extremely large. The successive whorls of the shell in the 

 female show a greater enlargement than is the case in male 

 shells. The lines marking the point of contact of two 

 successive whorls are known as the sutures. The mouth 

 of the shell is produced into a short anterior canal or 

 shell siphon (fig. 1, PI. I) in which lies a prolongation of 

 the mantle, the pallial siphon (fig. 7, Siph.). This con- 

 dition is characteristic of carnivorous gastropods. The 

 pallial siphon can be extended some distance out of the 

 shelly canal, into which it is retracted when the body is 

 withdrawn. In some gastropods there is also an anal or 

 posterior canal which is represented by the perforation in 

 Fissurella and the series of holes in Haliotis. 



The outer surface of the shell is covered by a horny 

 layer which can be stripped off quite easily. In worn 

 specimens it is frequently absent in patches. This layer, 

 the periostracum (PI. I, fig. 4) gives the shell a some- 

 what brown appearance and a furry surface. It will be 

 referred to in detail below. The external surface of the 

 whorls is marked by very distinct grooves and ridges 

 which run regularly in a longitudinal direction, and can 

 be traced round all the whorls to the apex of the shell. 

 They correspond to the lines radiating from the umbo of 

 a lamellibranch shell. In Buccinum these lines are 

 arranged on crests and troughs; and are in groups of 

 about six ridges between two crests. The crests become 

 much more distinct as one passes from the mouth towards 

 the apex of the shell, where they are more crowded. 



Running roughly at right angles to the former 

 system of longitudinal crests, and therefore transversely 

 to the direction of growth, is another system. This is 

 made up of two sets of markings — lines of growth and 



