90 PHOLADID^. 



affinity with Pholas than with Teredo, though at first 

 glance the shell bears most resemblance to the latter. The 

 tertiary fossil genus, Jouanettia^ is in some respects inter- 

 mediate between Teredo and Xylophaga^ of which no 

 extinct species are known. 



Xylophaga Dorsalis, Turton. 



Plate II. figs. 3, 4. 



Teredo dorsalis, Ti'rt. Conch. Diction, p. 1 85, (erroneously as to tube) ; Dithyr. 

 Brit. p. lG,pl. 2, f. 4,5. 



Xylophaga, Turt. Dithyra Brit. p. 253. — Flem. Brit. Anim. p. 455. — Brit. 



Marine Conch, p. 32. — Annals of Nat. Ilist. Septem. 1847. — 

 Brown, 111. Conch. G. B. p. 117, pi. 50, f. 8 to 13.— Sow- 

 BRBY, Genera Shells. — Rkeve, Conch. Syst. vol. i. pi. 22. — 

 Sow. Conch. Manual, f. 50,51. — Hanl. Recent Shells, p. 

 10, supp.pl. II, f, 21,22. 



Pholas aylophapa, Desh. Lam. Anim. s. Vert. ed. 2, vol. vi. p. 47. 



Xyluphagc dorsal. Ciienu. Traite Elem. p. 40. f. 122, 123. 



A hasty glance at this interesting and uncommon shell 

 would induce us to place it with Teredo., and the closing of 

 its posterior side, where there is not the slightest gape, will 

 probably be the first differential point to attract our atten- 

 tion to its generic as well as specific distinctness. The 

 general shape (allowing for the subrectangular incision at 

 the anterior side) is subrotund, the distance from the ex- 

 treme points of the front and hinder margins being at least 

 equal to that from the umbo to the opposite edge. The 

 texture is remarkably fragile, devoid of colour, and but 

 little glossy, even when the specimens are fresh. The valves 

 are tumid, but not quite hemispherical, and strengthened 

 internally by a narrow, but strong, prominent, somewhat 

 jointed rib, which, running perpendicularly from the umbo 

 to the ventral margin, divides the shell into two very un- 

 equal portions, the hinder being thrice the bigness of the 

 anterior one. The surface is externally marked with two 



