THE OPERCULUM. 



445 



and the muscle of attachment only moves nearer towards its 

 anterior edge as the addition of new matter on that side 

 renders such a displacement necessary in order to keep the 

 muscle, which enlarges in proportion, in its proper situation 

 with regard to the pillar of the shell. 



The subannular opercula (Fig. 79) may be regarded as 

 intermediate between the annular and spiral forms, partially 

 combining the characters of each ; but I think it better to 

 consider them apart from the others, inasmuch as they are 

 peculiar to those animals of the Ctenobranchous Mollusca, 

 which are provided with a siphon in front of the mantle 

 for conducting the water to their branchiae, such as the 

 Murices, Buccina, Strombi, Melanise, Melanopsides, Au- 

 lodi, and the anomalous genus Phorus. They are all of 



Fig. 79. 



a horny texture, and are characterised by their very de- 

 pressed cone being somewhat oblique, with its nucleus 

 placed at or near the left end, and the lines of growth form- 

 ing more or less complete rings around it, but always be- 

 coming wider apart from each other as they approach the 

 right ^side. The left end, towards which the nucleus is 

 placed, is generally acute, and the opposite extremity 

 rounded, which is just the reverse of what takes place in 

 the annular opercula, where the right end is acute and fur- 

 nished with a fold proceeding from the nucleus, and the 

 left side is rounded and broad. In most of the opercula 

 of this division the muscular scar occupies the greater part 

 of the internal surface ; is marked with more or less regular 



