68 The Shipzvorm. [Sess. 



Britain is Teredo norvegica. Teredo navalis occurs on all the 

 "western and southern coasts of Europe, from Christiania to the 

 Black Sea, and is the species causing so much damage to the 

 Dutch embankments. Fourteen species are said to live between 

 low-water to more than 100 fathoms in Norway, Britain, and 

 the Tropics. Species are also known in the fossil state, both 

 by their shells and burrows. The genus is said to have com- 

 menced in the Lias, and is well represented at the present day, 

 and in it and succeeding beds twenty-four species have been 

 discovered. 



The comparatively harmless Pholas, on the other hand — the 

 family type — whose working assists principally in the disinte- 

 gration of rocks, has its shell valves open at both ends. From 

 one of the ends the siphons protrude, while at the other is 

 seen the sucker-like disc or " foot." The valves are thin, 

 white, brittle, and covered with regular ridges of teeth. There 

 are also accessory valves, as in the specimens before you. The 

 siphons are often united almost to the end, and the outlet tube 

 indicates its function with a minute explosion in the water. 

 They are popularly known as Piddocks. 



It may not be out of place to refer in passing to two 

 other timber-boring pests — viz., the Limnoria terebrans and the 

 Chelura terebrans. The Limnoria terebrans, or wood - borer, 

 resembles a wood-louse, is not so rapid in its work as the 

 Teredo, but as certain with its results, and while the Teredo is 

 attacking the interior of a pile, the Limnoria may be destroy- 

 ing the surface. The late Robert Stevenson, who is credited 

 with the discovery of this crustacean during the building of 

 the Bell Rock lighthouse in 1810, established a regular series 

 of observations there, beginning in 1814, and tested the 

 ability of many kinds of timber, under like conditions, to 

 withstand this borer ; the result being that while it destroyed 

 most kinds, greenheart, beefwood, African oak and bullet- 

 tree were scarcely attacked, while teak and locust-tree stood 

 remarkably well. 



The Chelura or wood-boring shrimp is much more destruc- 

 tive than the Limnoria. It does not bore very deeply below 

 the surface ; and when the undermined portion is washed 

 away by the sea, it makes a fresh attack, and the timber is 

 thus destroyed in successive layers. The Teredo and Lim- 



