OTHER BIVALVE MOLLUSKS 107 



larger, a specimen before me measuring fully six 

 inches in length. The white shell is covered with a 

 brown periostracum, which in young shells is pret- 

 tily lined and almost glossy, while at the posterior 

 end it is decidedly ridged and wrinkled. The front 

 end of this shell is narrower than that of the last 

 species, and the pallial sinus is smaller. 



Spisula falcata^ Gld., the Falcate Mactra, has a 

 shell of moderate size, resembling Mactra califor- 

 nica^ but it has a narrower anterior end and a smaller 

 sinus. The shell is glossy, thin, and rather flat. It 

 is found in British Columbia, but it extends far 

 southward. 



Labiosa undulata^ Gld., the Wavy Rseta, is a very 

 distinct, though rather rare southern species. The 

 hinge resembles that of the Mactras and shows the 

 triangular pit plainly. The outline is not very dif- 

 ferent from that of Figure 83, but the shell is very 

 thin, not merely at the edges but throughout its 

 whole extent, and it is very decidedly marked with 

 concentric ridges, which even appear on the inner 

 surface. Most clams as they grow not only add 

 to the edge of the shell but also thicken the older 

 parts from the inside, but the Wavy Rseta does noth- 

 ing of the kind, and it is no wonder that good speci- 

 mens of so large and so thin a shell are quite rare. 

 Near the beaks it is much inflated. Externally it 

 is light brown or ashy, and when at all thickened 

 the interior is white. Three or four inches is the 

 common length of grown specimens. 



The last member of this group has the largest 

 shell of all, sometimes reaching a remarkable length, 



