Ill 



slightly flexaoas or nearly straight ; the posterior lateral margin is more con- 

 vex, and eventually sweeps rapidly to the anterior side, its junction with 

 which forms an acute or narrow ventral termination. The beaks are very 

 narrow, tuberculated, and elevated. 



The surface externally and internally, like the other species, is ivory white 

 and somewhat polished. The anterior area is concentrically sculptured, he- 

 coming more crowded towards its base ; it is separated by a slightly impressed 

 line from the body. The body is ornamented by a radiating narrow area, in- 

 creasing towards the base, both sides defined by a furrow. This area is sub- 

 divided into two, and is closely transversely striated, and marked less fre- 

 quently by minute raised ridges, directed obliquely downwards to the centre 

 from each outer margin. The surface of the fang and auricle posterior to the 

 radiating area, is smooth or sparingly striate. The auricle is not separated 

 from the body by any marked line, but its commencement is marked by the 

 transition from a convex to a concave surface, caused by the great outward 

 reflexion of the a\iricle. 



Internally, the beak is small but prominent, bearing a narrow oblique rib 

 on its surface, and turned posteriorly. The apophysis hugs rather closely to 

 the body, and i6 thin, blade-shaped and acuminated to the end. The ventral 

 tubercle is well formed and conspicuous but does not exhibit much evidence 

 of arising from an internal rib. The auricle is not internally defined, save by 

 Ihe greater thinness and translucency of its substance. 



The pallets are small, the blades are somewhat heart shaped at the apex, 

 rounded and curved outwards to an extreme point near their base on either 

 side. These points are not opposite, but one is situated higher on the blade 

 than the other ; from these the margins concavely contract into the stalks, 

 which become narrower towards their termination, ending in a point. 



Tube solid, not concamerated, twelve to eighteen inches long. Diameter ot 

 valves about half an inch. Mr. Hanley described this species in the British 

 Mollusca, supposing it to be identical with Turton's T. n a n a , whose name 

 and description he suppressed on the ground that they were founded on young 

 and imperfect shells. It has since been ascertained that this species is dis- 

 tinct from T. nana. 



Messrs. Fischer and Jeffreys both consider T. d i 1 a t a t a of Stimpson, a 

 synonym of this species, but the concamerated tube and differently formed 

 pallets are prominent distinctive characters. 



The blade of the pallet in d i 1 a t a t a dilates convexly from a very fragile 

 stalk into a broadly oval form, truncate at the end, while inT. megotara 

 the dilation is concave to a point on each side, from which the margins are 

 narrowed and rounded to a bilobed truncated end. The pallets ofdilatata 

 are more nearly allied to those ofNorvagica than to m e g o t a r a. 



T. nana, Turton. 



T. nana, Turton, Conch. Dithyra, p. 16, t. 2, f. 67. Adams, Genera, ii. p. 

 333. Brown Conch. Gt. Brit. p. 116, t. 50, f. 14, 15. Catlow, Conch. 

 Nomenc. p. 3. Fleming, Brit. Anim. p. 455. Gray, Phil. Mag. 1827, 

 p. 410. Hanley, Desc. Cat. p. 4, t. 11, f. 22. Jeffreys, Ann. & Mag. 

 Nat. Hist. 3d ser. vi. p. 122. Thorpe, Brit. Mar. Conch, p. 29. 

 T. nana, (part.) Fischer, Journ, Conchyl. 2d ser. i. p. 136. Gray, Ann. 

 & Mag. Nat. Hist. 2d ser. viii. p. 386. 

 T. navalis, Moller, Moll. Green. 

 T. denticulata, Gray, Ann. & Mag. Nat. Hist. 2d ser. viii. p. 386. Adams, 



Genera, ii. p. 333. Fischer, Journ. Conchyl. 2d ser. i. p. 135. 

 Pholas Teredo, Muller, Prodr. Tool. Dan, p. 251. Fabricus, Faun. Groen. 



p. 427. 

 Hab— England. "Floating wood. Occurs with T. megotara and 

 eubericola, but by no means so numerous as either of them." Jeffreys, 

 Northern Ocean. Greenland. 



** 



