116 PHOLADID.E. 



Teredo. This hypothesis seems preferable, in a bio- 

 logical point of view, to that of Deshayes, who likened 

 the appendage in question to the sheath of Teredo. 

 Very few species of Pholadidea are known ; and only 

 the typical species (P. papyracea) is fossil. 



Pholadidea papyra'cea*, Turton. 



Pholas papyracea, Turt. Conch. Dith. p. 2, t. i. f. 1-4. Pholadidea pa- 

 pyracea, F. & H. i. p. 123, pi. v. f. 3-6, pi. ii. f. 1, and (animal) pi. F. 

 f.4. 



Body somewhat conical, bluish-white, mottled in the centre 

 with white roundish spots : tubes, when fully extended, often 

 twice as long as the shell is broad, at other times more or less 

 strongly wrinkled across ; orifice of larger tube encircled by 

 about 20 white cirri of different lengths ; sheath of a pale 

 reddish-brown hue, terminated by a fringe of short white 

 cirri : foot clear white or almost transparent : liver green. 



Shell convex, thin, and of a delicate texture, depressed in 

 the middle, and divided into two nearly equal parts by a rather 

 narrow groove or constriction, which runs obliquely from the 

 beak in each valve to the front margin : sculpture, numerous 

 transverse scalloped ridges on the upper half of the anterior 

 side of the groove, the lower half being nearly smooth, much 

 thinner, and forming an oval-shaped dome ; the crests of the 

 ridges are sometimes prickly but not much raised ; the pos- 

 terior half is marked only by irregular lines of growth : colour 

 dirty white : epidermis very thin, partly fibrous at the pos- 

 terior end, light yellowish-brown : margins rounded (in the 

 young obtusely angular) on the anterior side, straight (in the 

 young widely gaping) in front, squarish (in the young rounded) 

 at the posterior end ; anterior dorsal margin upturned, doubled, 

 and folded back ; posterior one pinched up and nearly straight 

 (in the young sloping, so as to give a wedge-like appearance 

 to that part of the shell) : beaks placed at a distance of about 

 -|ths from the anterior end : hinge-line flexuous : hinge- 

 plate extremely broad, folded over the anterior side, and form- 

 ing a free angular projection above that part of the hinge ; 

 from the posterior part of the hinge issues an oblique triangu- 



* Paper-like. 



