ARCA. 237 



ed by bringing together the edges of the mantle-lobes in 

 the branchial and anal regions. The mantle varies in 

 colour, either white or brownish or orange or yellow, and 

 has about forty equidistant black dots, obsolete anteriorly, 

 ranged along its edge. The foot is long, narrow, geni- 

 culated towards its base, and deeply-grooved, so as to form 

 a disk throughout its length, especially in the region of the 

 byssal groove : it is usually, as well as the body, of a flesh- 

 colour, sometimes yellowish-white. The byssal groove is 

 ample and forms a mould for the filamentous matter, which 

 is cast in shape of a thin cup-shaped membrane, eventually 

 forming a thick and firm brown operculum, which on being 

 closely examined is found to consist of the combined byssal 

 threads. This operculum can be cast off and reformed with 

 great rapidity. The branchiae are brown, long, and nar- 

 row ; their outer surfaces are less striated than the inner. 

 " There are no palps, strictly so called ; but the linear 

 branchiae, without terminating as usual, are continued in 

 the same narrow form around the buccal orifice, and meet 

 with their fellows on the other side, and thus form a pair 

 of true labia, rather than palpi " (Clark). 



This fine Ark was once esteemed a very rare British 

 shell, but has of late years been found in numerous locali- 

 ties all round our coasts. It is taken in crevices of rocks, 

 in chinks of old shells, in the interior of dead shells, and 

 sometimes quite free ; when found in confined localities it 

 is often much distorted, but free specimens are very regular. 

 It is most abundant on the south-west coast of England, 

 the south and west of Ireland, and off the Zetlands. The 

 following selected localities will illustrate its range, in the 

 living state : Herm, near Guernsey, on dead oysters (S. H.); 

 Exmouth on oysters from deep water (Clark) ; in crevices 

 of rocks, Falmouth (Cocks) ; Penzance, in twenty fathoms 



