206 JOURNAL OF CONCHOI.OGY. VOL. I4, NO. 7, JULY, I914. 



3^in. by 2|in.,and another trawled 80 miles N.E. of Aberdeen. 

 S. rugosa L. — Korea Straits 30-54^, young only ('Sylvia') ! 

 The Rev. Frank Knight dredged a large specimen of this shell 

 at the mouth of the Clyde measuring i|in. by fin. It grows still 

 larger in the Arctic Seas, whence my collection contains a valve from 

 Spitzbergen 2in. broad by -^in. deep, and which must have been a 

 veteran, as it is abnormally thick and solid. 



S. rugosa is unusually variable, and there is no dividing line 

 between it and its varieties. Wliile some are sharply and others only 

 obtusely keeled, still others have not the slightest trace; some are 

 ridged across the valves, and in others these ridges are serrated, but 

 they all merge one into the other by imperceptible gradations. 



The immature forms are especially deceptive, both in their outward 

 appearance and in the nature of their habitat. They live in a free 

 state in the corallines of rock pools, instead of the rocky habitat of 

 their parents. They are depressed and inequivalve, and there is an 

 absence of any distortion, epidermis, or rugosity, until they arrive at 

 a stage when their specific instincts or ambition induce in them a 

 desire to excavate a more secure and permanent home, when they 

 seek the rocks, though in the absence of stone they will put up with 

 oyster shells. Thracia distorta is similarly deceptive, and has very 

 similar habits in its young stage, living in corallines and later seeking 

 a shelter, but not excavating, the burrows left by other molluscs in 

 oyster shells or stones, limestone by preference, and while the adults 

 are polymorphous in shape, the young are always regular in form and 

 outline. 



var. pholadis L.— An unusual specimen from Torbay has the 

 frontal gape, which distinguishes this variety from the type, closed as 

 in Pholadidea. 



Venerupis irus L. — Alderney, a valve (Marquand) ! It is 



said to have been found at Brodick, in Arran, by the Rev. J. E. 



Somerville. 



Petricola pholadiformis Lam. — This species has come to stay. 



]\Ir. T. Edwards in 1899 found it rather plentiful near Heme Bay, 



between tidemarks, about six inches below the surface, living in the 



Thanet sand, the lowermost strata of the tertiary group. Mr. Arthur 



Mayfield has recorded its appearance along the Suffolk coast, Mr. 



Gyngell from Lincolnshire, Mr. Sikes from the Medway, and Mr. 



Arthur Smith from Mablethorpe, Lines. See also a note by Mr. 



J. E. Cooper mjoitrn. of Conch., 1899, vol. ix., p. 243. 



Pholas dactylus L. — This may occasionally be dug up alive, at 



low spring tides, from the submarine forest bed on the Torre Abbey 



sands at Torquay. 



