1 70 INTRODUCTION TO ZOOLOGY. 



effort they stay 84 or 87 seconds.* The depth is commonly 

 from four and a half to eight fathoms. The entire amount of 

 revenue derived from the pearl-fisheries of Ceylon, from 

 March, 1828, to May, 1837, amounted, according to the 

 same authority, to 227,131, but has decreased very con- 

 siderably since that time. 



The large Scallop, or, as it is called in the North of Ireland, 

 the "Clam-shell" (Pecten maximus), can move rapidly through 

 the water by striking the valves of the shell together, and 

 thus propelling itself in the contrary direction. From their 

 lively movements in the water, and the vigorous flappings of 

 their brightly tinted valves, they have obtained the name of 

 sea-butterflies, t 



The common Mussel (Mytilus edulis) enjoys no such power 

 of locomotion, being moored to its "bed" by the silken cable 

 which it constructs for the purpose. This byssus, or, to use 

 a more common term, this beard, of the Mussel, has been 

 employed to assist in giving additional strength to works of 

 human construction. At the town of Biddeford, in Devon- 

 shire, there is a long bridge of twenty-four arches across the 

 Towridge river, near its junction with the Taw. At this 

 bridge the tide flows so rapidly, that it cannot be kept in 

 repair by mortar. The corporation, therefore, keep boats in 

 employ to bring mussels to it, and the interstices of the bridge 

 are filled by hand with these mussels. It is supported from 

 being driven away by the tide entirely by the strong threads 

 these mussels fix to the stonework; and by an act, or grant, 

 it is a crime liable to transportation for any person to remove 

 these mussels, unless in the presence and by the consent of 

 the corporative trustees. J 



The Pinna, a bivalve already mentioned (page 84) excels 

 any other in the quantity and fineness of its silk, " which has 

 been woven into some articles of dress, that in early times 

 were so highly prized as to be worn only by emperors and 

 kings." At Taranto, in Italy, it is still mixed with about one- 

 third of real silk, and made into gloves, caps, stockings, &c. 



* Pearl Fisheries of Ceylon, by James 'Stctiart, Master Attendant at 



Colombo, and formerly Inspector of Pearl Banks Printed at Ceylon, 



1843. 



f Owen, page 291. 



j Daniel's Rural Sports, vol. ii. pnge 00. 



