CHjiit-shells and Mussels 59 



insufficient room in a shell shaped like that of 

 L. pycpncui to withdraw the whole of the long 

 siphons. The valves at this point do not fit closely. 

 There is a slight gape, and this indicates that owing 

 to the constant protrusion of the siphons the mantle 

 is never able to extend the edges of the shell so 

 that they will meet. In some other bivalves we shall 

 see that the foot also is kept so constantly distended 

 that the shell gapes at the front end also. The 

 hinge-teeth are in this species less simple in their 

 shapes, as will be seen in the figure of the interior 

 of a left valve. It is found in muddy gravel and sand 

 between 20 and 100 fathoms deep, on our northern 

 coasts. 



The two genera Nuciila and Leda constitute the 

 sole British representatives of the order Pro- 

 tobranchiata, the bivalves in which the branchiae 

 or respiratory organs have the simplest character. 

 They consist of parallel, unbranched, independent 

 filaments arranged in two rows on each side of 

 the central body-mass. In the next lowest group, 

 the order Filibranchiata, 

 the filaments have become 

 so long that they are 

 folded back upon them- 

 selves and show signs of 

 lateral adhesion. The 

 order includes the Saddle- 

 oysters (Anomia), the 

 Ark-shells (Area), and the 



Mussels (3Iytilus). Saddle-oyste.-.nde.slde 



The Saddle-oyster {Anomia eplnppiwm) is a thin, 

 flat, little pearly shell found on old oyster-shells, crabs. 



