456 MOLLUSCA. PHOLAD^. Pholas 



PHOLAD^. 



Gen. CXXXIII. PHOLAS. Shell transverse; ligament 



slender, and covered by a reflected fold of the cloak, and 

 sometimes protected by calcareous plates; hinge with a 

 curved process under the margin in each valve, receding 

 into the cavity. 



* Valves divided by a longitudinal groove. 



553. P. crispata. — Transversely oblong, rounded, and gap- 

 ing retrally, obliquely truncated, and open anteally. 



P. latus, List. Conch, t. ccccxxxvi. An. Ang. 192. t. v. £ 38. Anat- 

 t. xix. £ 3 — P. crisp. Linn. Syst. i. 1111. Mont.- Test. Brit. 23. Turt. 

 Biv. Brit. 6. — In calcareous and argillaceous rocks. 

 Length 2 inches, breadth 3 ; white ; a groove runs from the hinge to the 

 middle of the ventral margin, where the valves come in contact, dividing 

 the shell into two compartments; the anterior is very rough, with numerous 

 thin waved concentric ridges, with obsolete longitudinal furrows ; the poste- 

 rior is comparatively smooth ; margin of the shell at the hinge reflected, 

 smooth, covered by a fold of the cloak, and strengthened by an obscure im- 

 bedded calcareous plate ; projecting tooth linear ; foot or sucker large, the sy- 

 phon tubes produced. 



554. P. lamellata. — An erect triangular plate at the hinge, 

 placed retrally with respect to the recurved tooth. 



P. papyracea, Turt. Biv. Brit. 2. t. i. £ 1-4 — T. lamellata, It. 4. t. i. 

 £ 5, 6. 



Length half an inch, breadth an inch ; white; anterior compartment witli 

 the waved ridges broken into thin denticular processes in bent longitudinal 

 rows; posterior compartment comparatively smooth ; the valves shut close 

 retrally from the mesial furrow ; behind the hinge, dorsally, the margin is 

 closed, compressed, and prominent ; before the hinge the margin is a little 

 open, and reflected on each side into a thin, nearly erect, plate ; the anteal ex- 

 tremity is obliquely truncated and open — This is the condition of the shell 

 when of a certain age (and constituting Dr Turton's P. lamellata.) It seems 

 afterwards to change its form ; the oblique truncated anteal aperture is filled 

 by the deposition of shell nearly smooth, bringhig the now tumid edges 

 nearly in contact ; retrally the shelly matter deposited, is broadest towards 

 the extremity, now become truncated and a little open, to which is attached 

 a coriaceous expanding cup, divided longitudinally, and thickened at the 

 margin dorsally and ventrally ; this seems destined to be a basal sheath tm\ 

 the syphons ; anteally the margins approximate, and are thick and elevated 

 before the hinge. The cloak is now closed, with only a small opening for 

 the foot. In this last state, Dr Turton has described this shell as the Pholas 

 papyraceus of Solander's MSS, and regards it as quite distinct from P. lamellata. 

 A comparison, however, in reference to their mode of growth, of specimens 

 of both shells, obligingly furnished to me by Mr G. B. Sowerby, has left no 

 room for hesitating regarding their specific identity. The structure of the 

 hinge, so different from the other species, and the singular changes of form, 

 with age, would justify the erection of the genus Pholadidia, originally con- 

 templated by Dr Goodall for its reception. 



