MYA. 67 



small, marked with a brown spot at the base of each ; valve 

 of excurrent tube conspicuous : gills pale brown, their points 

 entering the lower tube; 2xil2ys large, excessively thin, and 

 rather sharp - pointed : foot narrow and straight, yellowish- 

 white. 



Shell oval, less inequivalve than M. arenaria, nearly equi- 

 lateral, gaping widely at the posterior end but very little at 

 the anterior end, rather convex (especially towards the beaks), 

 solid, opaque, and lustreless : sculpture as in the last species : 

 colour greyish-white, with often a yellow or ochreous tinge: 

 epidermis rather thick, .irregularly wrinkled or puckered, and 

 minutely striated in a transverse direction : margins rounded 

 on the anterior side, nearly straight in front, and truncated 

 on the posterior side ; dorsal margins sloping equally on both 

 sides : heahs small, sharp -pointed and inflected, more or less 

 contiguous, and sometimes abraded by mutual pressure : car- 

 tilage, hinge-line, and hinge-plate as in M. arenaria ; but the 

 hinge-plate is narrower : teeth, in the right valve an oblique 

 spur-like cardinal, which is more conspicuous in young and 

 immatm-e specimens ; in the left valve a nearly upright trian- 

 gular plate, with a central cavity for the cartilage and a ridge- 

 like process or wall on the posterior side ; this plate is not so 

 large as in M. arenaria, compared with the size of the shell : 

 inside chalky -white, but occasionally nacreous and exhibiting 

 a few minute pearls within the pallial line: scars strongly 

 marked. L. 2. B. 2-65. 



Var. ahhreviata. Shell not so broad, abruptly truncated at 

 the posterior end. 



Habitat : Littoral in muddy gravel and sand ; but 

 frequenting more the open sea than M. arenaria. 

 It is sometimes found at considerable depths : I 

 dredged a young live specimen of the variety on the 

 Antrim coast in 80 f. about 10 miles from land. This 

 variety has also been taken by Professor King on the 

 Dogger bank, and by Mr. Barlee in Shetland. M. 

 truncal a occurs in every upper pliocene bed, including 

 Moel Tryfaen (Darbishire) ; boulder-clay at Wick, 

 Whitby, and Scarborough (Peach, J. G. J., and Leck- 

 enby) ; Sussex raised beach (Godwin- Austen) ; Norwich, 



