INTRODUCTION. XXX111 



worth their while to bring home what they could 

 collect, as an object of traffick. It is principally to 

 this cause that we owe the produce of the different 

 shores, and receive shells from India, Amboina, 

 Ceylon, New Holland, &c. which would never reach 

 England, if they were not profitable commodities. 



All marine shells are not confined to the depths of 

 the ocean, but, on the contrary, are often led, by an 

 instinctive faculty, to fix on rocks, or bury them- 

 selves in the sands. These latter may be discovered 

 on the smooth flat shores, while the tide is ebbing, by 

 little bubbles of air rising from small openings in 

 the sand. Each of these places denotes the retreat 

 of a marine animal, and frequently of a shell, which 

 may readily be dug out with a spade. The species 

 of the genus Tellina, Solen, and Mya, conceal them- 

 selves in this manner, besides some kinds of the genus 

 Buccinum, and many other smooth, univalve shells. 



If the rocks that are washed by the sea are found 

 pierced with small holes, regularly wrought, as if 

 they had been bored with an instrument, it is a 

 proof that they either are, or have been, inhabited 

 by Pholades, or by the rugged muscle (Mytilus ru- 

 gosus, Linn, j The only way to procure these, is to 

 break into their habitations with a hammer ; but, as 

 the shells are very brittle, it will be difficult, by 

 such means, to obtain them entire. 



