TEREDO. 169 



the posterior side, which is terminated by a semicircular ex- 

 pansion, usually termed an " auricle ; " in younger specimens 

 this auricle is entire, and has a high shoulder above, on a level 

 with the umbo, but in aged specimens the shoulder is worn 

 down by the continual attrition of that part, and a notch is 

 formed above ; dorsal margins sloping abruptly and equally 

 on each side : beaks much incurved, situate near the anterior 

 end, at about one-third the length of the dorsal line ; umbones 

 or rostral portion prominent : hinge-line angular and irregular, 

 considerably projecting in the middle : hinge-plate very broad, 

 and extremely thick, folded over the anterior dorsal area, and 

 abruptly truncated and flattened, or occasionally excavated, 

 on the other side ; the centre is furnished with a callous 

 protuberance, as well as with a short peg-like tooth or 

 process, which is stronger and more conspicuous in the right 

 than in the left valve: apophyses very broad, and of ten jagged 

 at the edges : inside glossy, furnished in front with a rather 

 large and solid pear-shaped excrescence, and having the pos- 

 terior auricle separated by a strong ridge, which forms a 

 shelf or ledge in aged specimens : muscular scars large but not 

 strongly marked: pallets large; blades oval, wedge-shaped 

 and truncated or squarish in front, somewhat convex outside 

 and concave inside, of a laminated structure, and more or less 

 covered (especially at the outer end) with the same kind of 

 epidermis as invests the shell; stalks cylindrical, of a much 

 more solid substance than the blades, varying in length, being 

 usually about one-third the length of the blades; the stalk 

 occasionally extends into the blade at its narrower or inner 

 end, and appears like the midrib or nerve of a leaf: sheath 

 thick, sometimes indistinctly annulated ; septa or plates in the 

 neck of the sheath broad and imbricated outwards ; they are 

 divided near the opening of the sheath by a sharp ridge on 

 each side, which separates the branchial and excretal tubes of 

 the animal, and is continuous in perfect specimens, so as to 

 form two distinct holes. Yalves, L. 0-6, B. 0-65; pallets, 

 L. 0-8, B. 0-3; sheath, L. 12, B. 075. 



Yar. divaricata. Shell stunted, distorted, and thicker, having 

 the anterior area much more developed than usual, and scarcely 

 any posterior auricle. T. divaricata (Deshayes, MS.), Fischer, 

 in Journ. Conch, v. p. 137, pi. vii. f. 7-9. 



HABITAT : In oak, fir, and birch wood composing the 

 timbers of sunken vessels, piers, shipping-stages, and 



VOL. III. I 



