CONCHIFEEA. 39 



the ends of certain muscles, and by a horny skin 

 (epidermis) belonging to the mantle, and stretch- 

 ing over the edges of the valves and upon their 

 exterior. Sometimes the epidermis encloses even 

 the siphons, but in many cases it is not to be 

 detected at all. 



At the back the valves are connected together 

 partly by an elastic dense substance, somewhat 

 resembling india-rubber (the ligament], and partly 

 by a hinge, usually made of teeth and cavities 

 fitting into each other. The use of the ligament is 

 to force open the valves, in opposition to the con- 

 tractile power of the muscles within, which draw 

 them together. The ligament may be placed within 

 or without the hinge, or partly the one and partly 

 the other. In the former case the fibres are com- 

 pressed, and in the latter they are stretched when 

 the valves are closed ; in either case it is their 

 elasticity which opens the valves. 



On separating the valves, we see next the two 

 leaves of the mantle. These are delicately thin, 

 except at their external edges, where they are 

 somewhat thickened. These margins, also, are fre- 

 quently furnished with sensitive tentacles and 

 other organs of sense, as well as with glands, 

 which are often highly coloured. One prominent 

 function of the mantle is the formation and peri- 

 odical increase of the shell, the process of which 

 has been so graphically described by Professor 

 Jones that I shall cite his words : 



" When the animal is engaged in increasing the 

 dimensions of its abode, the margin of the mantle 

 is protruded, and firmly adherent all round to the 

 circumference of the valve with which it corre- 

 sponds. Thus circumstanced, it secretes calcareous 



