104 



MADRAS FISHERIES BULLETIN 



VOL. XIV, 



In the Ear-shells, although the general shape is flattened, the 

 essentially spiral form is clearly apparent at the hinder end where a 

 distinct short spire is seen. In the vast majority of gastropods this 

 spire is greatly emphasized and in the next family to be described, 

 the Top-shells, this form is fully established. Such molluscs are 

 said to be highly asymmetrical, but this lack of symmetry is con- 

 fined almost wholly to the visceral mass — the part containing the 

 viscera — the stomach, intestine and so-called liver ; the mantle, the 

 shell and the gills are also affected by this twisting. The foot and 

 the head usually remain as symmetrical as in the Chitons. In other 

 words the " back " of these animals has been twisted and coiled 

 into a spire, and the shell takes on the same shape, as it is formed 

 by shell-forming secretions poured out by the fold of integument 

 that covers the viscera. 



TheTOF-SHEhLSiTrochidie) and the TURBAN-SHELLS {Turbi- 

 nidae) may conveniently be taken together. They are much alike in 

 appearance, both comprising some of the commonest of our littoral 

 shells. In shape they are conical and are beautifully pearly within. 

 The larger kinds are commercially valuable, for the widely spread 

 " Trocas " shell of trade {Trochus niloticns) and the even more 

 valuable "Green snail" {Turbo mnrnwratus) of eastern seas are 



Fig. 2. The Comaion Tiirhan shell (7'. :!r<;}ros/owa) with inner 

 and outer views of the operculum X I. 



in great demand for pearl button manufacture. The two families 

 are easily distinguished by the nature of the operculum, a flattened 

 hard structure found upon the upper surface of the tail end of the 



