122 TEREDINID^E. 



This curious little mollusk attacks and injures sub- 

 marine timber, but not to anything like the extent that 

 Teredo does. Its burrow only extends \\ inch in 

 depth. The course of its perforation is diagonal or 

 slanting, and therefore is partly 'against the grain of 

 the wood. Its cell is flask-shaped with occasional con- 

 cavities, the edges of which are sometimes sharp to 

 receive the sides of the shell during the progress of the 

 animal. 



It is the Pholas xylophaga of Deshayes. 



Family XXIV. TEEEDI'NID^, Fleming. 



BODY worm-shaped and almost gelatinous, more or less 

 enclosed in a testaceous sheath, which is usually flexuous: 

 mantle very thin and cylindrical, enveloping the whole body, 

 open only for the passage of the foot at the anterior end, and 

 for the orifices of the tubes or siphons at the posterior end ; 

 it is folded back over the hinge of the shelly valves at the 

 anterior end, as in the Pholadidce ; and it adheres to the sides 

 of the sheath at the base of the pallial tubes, by means of a 

 muscular ring : these tubes are short in proportion to the 

 length of the body, but extensile ; they are united near their 

 origin, and forked towards their extremities ; orifices fringed 

 with short cirri : gills, a pair on each side, long, ribbon-like, 

 and distinctly laminated ; they are separate in front, adherent 

 for the greater part in the middle, and prolonged behind to 

 the base of the larger tube: palps consisting of two pairs, 

 short and pectinated : foot large, truncated, muscular and 

 expansile, not byssiferous; it is attached to the rest of the 

 body by a thick and powerful cylindrical stalk. 



SHELL or principal valves placed at the anterior extremity of the 

 animal, helmet-shaped, equivalve, the valves touching each other 

 only at the hinge and in front, but elsewhere widely gaping ; 

 each is divided and sculptured as inXylophaga: epidermis mem- 

 branous and thin : beaks not prominent, when viewed in front, 

 owing to their being inflected : hinge connected by the anterior 

 adductor muscle, which supplies the place of a cardinal liga- 

 ment ; it is covered by a thickened fold of the mantle, but 

 there are no shelly plates or shields, such as the Pholadidce 



