History of British animals - University of Guam Marine Laboratory

456 MOLLUSCA. PHOLAD^. PholasPHOLAD^.Gen. CXXXIII. PHOLAS. Shell transverse; ligamentslender, and covered by a reflected fold of the cloak, andsometimes protected by calcareous plates; hinge with acurved process under the margin in each valve, recedinginto the cavity.* Valves divided by a longitudinal groove.553. P. crispata.— Transversely oblong, rounded, and gapingretrally, obliquely truncated, and open anteally.P. latus, List. Conch, t. ccccxxxvi. An. Ang. 192. t. v. £ 38. Anatt.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 themiddle of the ventral margin, where the valves come in contact, dividingthe shell into two compartments; the anterior isvery rough, with numerousthin waved concentric ridges, with obsolete longitudinal furrows ;the posterioris comparatively smooth ;margin of the shell at the hinge reflected,smooth, covered by a fold of the cloak, and strengthened by an obscure imbeddedcalcareous plate ;projecting tooth linear ;foot or sucker large, the syphontubes 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 witlithe waved ridges broken into thin denticular processes in bent longitudinalrows; posterior compartment comparatively smooth ; the valves shut closeretrally from the mesial furrow ; behind the hinge, dorsally, the marginisclosed, compressed, and prominent ; before the hinge the marginis a littleopen, and reflected on each side into a thin, nearly erect, plate ; the anteal extremityis obliquely truncated and open—This is the condition of the shellwhen of a certain age (and constituting Dr Turton's P. lamellata.) It seemsafterwards to change its form ; the oblique truncated anteal aperture is filledby the deposition of shell nearly smooth, bringhig the now tumid edgesnearly in contact ; retrally the shelly matter deposited, is broadest towardsthe extremity, now become truncated and a little open, to which is attacheda coriaceous expanding cup, divided longitudinally, and thickened at themargin dorsally and ventrally ; this seems destined to be a basal sheath tm\the syphons ; anteally the margins approximate, and are thick and elevatedbefore the hinge. The cloak is now closed, with only a small opening forthe foot. In this last state, Dr Turton has described this shell as the Pholaspapyraceus of Solander's MSS, and regards it as quite distinct from P. lamellata.A comparison, however, in reference to their mode of growth, of specimensof both shells, obligingly furnished to me by Mr G. B. Sowerby, has left noroom for hesitating regarding their specific identity. The structure of thehinge, so different from the other species, and the singular changes of form,with age, would justify the erection of the genus Pholadidia, originally contemplatedby Dr Goodall for its reception.