120 PHOLADID.^. 



Xylophaga dorsa'lis^ Turton. 



Teredo dorsalis, Turt. Conch. Diet. p. 185. X. dorsalis, F. & H. i. p. 90. 

 pi. ii. f. 3, 4. 



Body white, with the exception of the foot, which is tinged 

 with buff at its extremity. 



Shell helmet-shaped, convex, thin, parted in the middle 

 (but not equally, owing to the wide anterior gape) by a broad 

 longitudinal groove, which is margined on each side by a sharp 

 narrow ridge : sculpture as described in the generic characters ; 

 the striae which cover the anterior and middle areas, as well 

 as their interspaces, are exquisitely crenulated or crossed 

 obliquely by still more numerous and microscopical striae 

 (giving the edges of the main striae an exquisitely beaded ap- 

 pearance) ; these main striae become more crowded or close-set 

 as the growth of the shell increases, being at first comparatively 

 few and remote ; there is a distinct line of demarcation between 

 the two sets of main striae ; the marks of growth on the pos- 

 terior area are concentric and tolerably regular : colour white : 

 epidermis yellowish-brown, more persistent on the anterior 

 side of the separating groove : margins obtusely angular on 

 the upper part of the anterior side, with a large triangular 

 excision on the lower part, so that when the valves are united 

 the opening is broadly heart-shaped ; they are curved in front 

 with a notch for the groove, and rounded at the posterior end ; 

 dorsal margins sloping abruptly and equally on each side : beaks 

 much incurved, somewhat nearer to the anterior end : hinge- 

 line projecting and pointed in the middle, by reason of the 

 abrupt inflexion of the beaks, with a deep curve on either side : 

 hinge-plate very broad on the anterior side, over which it is 

 folded, adhering to the umbonal area but free towards the 

 extremity, where the edges are turned up ; it is narrow in the 

 middle and on the posterior side : apophyses curved and pro- 

 jecting outwards ; that of the right valve is larger than the 

 other ; in aged individuals they are thick and tusk-like : 

 dorsal shields not unlike the opercula of Neritina Jluviatilis, 

 but having a less decided spire and doubled underneath at 

 the wider end ; they lie close to the beaks, on the outside of 

 the dorsal anterior margin : inside glossy, marked with a broad 

 and strong rib, which corresponds to the external groove, and 

 sometimes also with a slight and indistinct ridge, which is 



* From its being furnished with plates on the back. 



