36 G. 0. Sårs. 



The shell of the male appears on the Avhole narrower 

 than that of the female, and has the edges less regularly 

 curved. Seen from the side (fig. 3), it exhibits a somewhat 

 elliptical form, with the height not nearly attaining 2 /3 of 

 the length. The dorsal margin here too, is straight and 

 horizontal, but the angle between it and the free posterior 

 edges of the valves is nearly obsolete. The umbones are 

 rather prominent, and occupy the same place as in the shell 

 of the female. The inferior edges of the valves are less 

 curved than in the female, being almost straight in the 

 middle, and of the extremities, the anterior one appears 

 bluntly and somewhat obliquely truncated, whereas the 

 posterior one is still more prominent than in the female, 

 with the tip evenly rounded. Seen from above (fig. 4), the 

 shell appears less tumid than in the female, though of a 

 similar irregular fusiform shape. 



The surface of the valves in both sexes is sculptured 

 by a number of very distinct concentric ribs, representing 

 the lines of growth. These ribs, the number of which of 

 course increases with age, may in fully grown specimens 

 amount to from 20 to 24 in each valve. They are distinctly 

 elevated, ridge-like, and placed at rather regular intervals, 

 the 1st one encircling the umbo, the last running along 

 the free edge of the valve. In front, the ribs converge to 

 one point in close approximation to the umbo, whereas 

 behind they terminate at some distance from each other 

 along the dorsal margin, giving it a more or less distinctly 

 crenulated appearance. On a closer examination, the greater 

 number of the ribs are found to be fringed in their posterior 

 part with short, flattened bristles, and on the outermost ribs 

 this fringe is very conspicuous, as it occupies the greater 

 part of their length (see fig. 3 a). The surface of the shell 



