THE THOKNY WOODCOCK. 



595 



scribed before proceeding further, inasmuch as its shape and compar- 

 ative dimensions often afford valuable indications by which a species, 

 or even a genus, may be distinguished. This structure is called " oper- 

 culum," and its use, when fully developed, is to close the aperture of 

 the shell when the animal has withdrawn itself into the recesses of its 

 home. 



The operculum can well be seen in the water-snails, where it attains 

 its full size and exactly fits the opening which it is intended to protect. 

 The material of which the operculum is essentially composed is a horny 

 substance, but in some species the horn is strengthened by layers of the 

 same nacreous matter which lines the shell, and becomes so thick and 

 heavy that when found separate from its owner it is often mistaken for 

 some species of shell. The operculum is very variable both in its 

 form and comparative dimensions, and even in its presence or absence. 

 Sometimes it is circular, like a flat plate, and composed of concentric 

 circles, while in some species it assumes a regularly spiral form like a 

 flattened watch-spring. 



The shells that are included in the family of the Muricidse may read- 

 ily be distinguished by the straight beak or canal in front, and the ab- 

 sence of any such canal behind. All the animals belonging to this 

 family are not only carnivorous, but rapacious, preying on other mol- 

 luscs, and destroying them with the terrible armature called the tooth- 

 ribbon, which, when examined with the microscope, proves to be a set 

 of adamantine teeth, sharp-edged and pointed as those of the shark, and 

 cutting their way through the hard shells of their victims as the well- 

 known cordon saw passes through thick blocks of hard wood. 



About one hundred and eighty species are known to belong to the 

 typical genus, and 

 there is hardly a 

 portion of the world 

 where a Murex of 

 some kind may not 

 be found. 



The illustration 

 represents the shell 

 which is popularly 

 known under the 

 name of Thorny 

 Woodcock, the lat- 

 ter title being given 

 to it, in common with several of its congeners, on account of its long 

 beak, which is thought to bear some resemblance to that of the wood- 

 cock, and the former in allusion to the vast number of lengthened 

 spines or thorns which are arranged regularly over its surface. It 



The Thorny Woodcock {Murex tenuv^pinis). 



