THE TEEEDO. 



251 



FAMILY XX1IL PHOLADIDJE. 



The characters of this family are, shell free, or within a tube ; valves equal, gaping at both ends, 

 thin, white, brittle, armed in front with rasp-like imbrications, without hinge teeth, and streng- 

 thened externally by accessory valves ; hinge-plate reflexed over the beaks, and furnished with 

 a long, curved muscular process 

 beneath each; anterior muscular 

 impression on the hinge-plate, 

 pallial sinus very deep. It lives 

 perpendicularly in holes in the 

 rock or sand. The Pholadidce 

 perforate all substances that are 

 softer than their own valves 

 (M. Cailliaud), but Mr. Hancock 

 has pointed out that the foot 

 appears to be a more efficient 

 instrument than the shell for 

 burrowing into rock, seeing that 

 its surface can be renewed as 

 fast as it is worn away. 



Genus Pholas. The com- 

 mon Piddock is \ised for bait 

 on the coast of Devon ; its 

 foot is white and translucent 

 when fresh. It has two accessory 

 valves to protect the umbonal 

 muscle, "with a small transverse 

 plate behind ; a long unsym- 

 metrical plate fills up the space 

 between the valves in the dorsal region. Thirty-two species are found living at twenty-five 

 fathoms. It is almost cosmopolitan. P. costata is sold as food in the market of Havannah. 



Genus Pholadidea. This genus resembles Pholas, but has a deep transverse furrow across 

 the centre of its valves; the anterior gape is large, but closed in the adult by a callous plate- 

 Seven species are found, from low tide to ten fathoms, in Britain, New Zealand, and Ecuador. 

 P/ioladidea and its sub-genera burrow into shell, wood, resin, wax, c. 



Genus Xylophaga. This genus bores into floating wood and timbei-s which are always covered 



by the sea. Two 

 species are living 

 in Norway, Britain, 

 and South America. 

 Genus Teredo. 

 The shell is globose, 



TEREDO NAVALIS. g a P m g anteriorly, 



and behind ; the 



valves are trilobate, concentrically striated, divided by a single transverse groove ; the hinge margins 

 are innexed anteriorly ; the interior of the valves is furnished with a long, curved process for the 

 attachment of the pedal muscle. T. navalis is ordinarily a foot long, sometimes two feet and a half; 

 it destroys soft wood rapidly, and teak and oak do not escape ; it always bores in the direction of the 

 grain, unless it meets another Teredo. In 1731-2 it did great damage to the piles in Holland, and 

 caused still more alarm ; metal sheathing and broad-headed iron nails have been found most 

 effectual in protecting piers and ship-timbers. The Teredo was first recognised as a bivalve mollusc 

 by Sellius, who wrote an elaborate treatise on the subject in 1733. (Forbes.) Fourteen species 

 occur living from low water to more than 100 fathoms, in Norway, Britain, and the Tropics. 



HENRY WOODWARD. 



ritOLAS DACTYLVS IN A SHELTER HOLLOWED OUT BY IT IN A liLOCK OF GNEISS. 



