280 MOLLUSCA FROM THE CRAG. 



This species I have not as yet seen from the Lower or Coralline Crag ; and although 

 abundant in the Red Crag, I have never found anything but detached valves, and 

 those in the more disturbed portion of that Formation. 



It is subject to great variation ; and I think, with the authors of the ' Hist, of Brit. 

 Mollusca,' that the shell called lata by Mr. Sowerby, above referred to, is only a 

 modified form of this species, although presenting an intermediate character between 

 the truncated posterior of truncata and pointed termination of the ordinary form of the 

 living shell, depending probably upon some peculiarly local conditions, as I have never 

 met with it but in one locality. The form of the spatulate tooth in the hinge of this 

 variety is precisely like that of the recent arenaria. It was furnished with a large and 

 strong ligament, or rather cartilage, the greater part of which is preserved in most of 

 the fossil specimens. 



The cause assigned for this variation (lata), by the authors of the ' Hist, of Brit. 

 Mollusca/ appears to me to be somewhat doubtful, this not being, I believe, one of 

 the forms found in the estuary portion of the Mammaliferous Crag, where distortions 

 are by no means rare, and where, in all probability, the influx of ice or the efflux of 

 more than ordinary quantities of fresh water, produced deformities like some of the 

 specimens of Purpura lapillus and Littorina litlorea, found in that Deposit. The 

 variation in lata does not appear to be a distortion, as understood in this case, but a 

 character that pervaded a whole race, making it what is called a permanent variety. 



The depth of the palleal sinus is a character here not to be much depended upon , 

 as, in my large series, considerable differences in regard to depth may be observed ; 

 for in some specimens this mark extends considerably beyond the hinge ligament, 

 while in others it falls short of it, being modified by the length of the tubes, which 

 would probably vary under the influence of external conditions. 



This, in the living state, is generally a very shallow- water species, burying itself in 

 sand, near low-water mark ; extending, at times, into rivers as far as where the water, 

 when the tide is out, is nearly fresh. Its geographical distribution takes in the whole 

 circuit of the Northern Hemisphere, being found, according to Middendorff,* at Sitka, 

 in the Sea of Okhotsk, on the coast of Russian Lapland, and Nova Zembla, and, by 

 the American authors, on the soast of the United States, as far to the southward as 

 nearly to 40°, exhibiting thus an equal, or perhaps a greater, extent of range than its 

 elder confrere, truncata. It has not had, however, on our side of the Atlantic, quite 

 so great a range to the southward, not having been found, either recent or fossil, in or 

 near the Mediterranean. 



The animal of this species is, according to Dr. Gould, extensively employed as bait 

 in the cod fisheries of Newfoundland, and is called the long clam, to distinguish it 

 from the giant clam, Mactra gigantea, or the round clam or Quahog, Venus mercenaria. 



'• The specimen figured by Middendorff appears rather distorted, with a short siphonal side. 



