66 PECTIXID.E. 



the byssal sinus is finely toothed : heals very prominent and 

 considerably raised : ears unequal ; those on the left hand of 

 the upper valve and right hand of the lower valve are at least 

 four times the size of the others; all of them have a few 

 strong ribs which radiate from the beaks, and they are also 

 marked with coarse and numerous striae which cross the ribs 

 obliquely; the ears of the lower valve project very little be- 

 yond those of the upper valve ; byssal notch deep : hinge-line 

 straight: cartilage small: ligament very thin: hinge-plate 

 narrow, microscopically striate across ; transverse rib strong 

 and considerably raised on each side of the cartilage -pit : in- 

 side nacreous, finely and closely striate lengthwise and notched 

 on the front margin ; within the larger ears are strong grooves 

 which correspond with the outside ribs : muscular scars dis- 

 tinct, especially in aged specimens. L. 0*9. B. 0-885. 



Yar. costata. Shell stronger than usual, and having either 

 five ribs and intermediate small ones, or else several riblets 

 of equal size : inside grooved and striate accordingly. 



Habitat : Xot uncommon from Shetland to the 

 Channel Isles in 7-82 fathoms, on a sandy bottom mixed 

 with gravel ; and the variety is equally diffused. This 

 species occurs as an upper tertiary fossil in the Scotch 

 glacial deposits, and also in the Red and Coralline Crag. 

 It is a native of the Icelandic and Scandinavian coasts ; 

 and Mr. AT' Andrew has taken it in Vigo Bay at a depth 

 of only 8 fathoms. 



The shell varies greatly in size and proportions, as 

 well as in colour and the number of ribs. My largest 

 specimens are one inch and two-tenths long, and nearly 

 the same in breadth. Sometimes the breadth slightly 

 exceeds the length. Young shells are proportionally 

 longer than old ones. In the former the so-called "eyes" 

 are perceptible through the semitransparent shell many 

 years after the animal has ceased to exist. The fry are 

 not sculptured, but perfectly smooth; and the lower 

 valve is nearly flat and smaller than the other. M tiller 

 noticed the way in which he presumed this scallop pro- 



