CLASS LAMELLIBRANCHIATA, 6^ 



beak turns away is the longest and narrowest, and is the 

 side at which the vent is situated (the posterior or hinder 

 side, fig. 26, p). 



Internally, the shell exhibits several points of import- 

 ance. Placed beneath the "dorsal" margin of the shell 

 (that is, the margin on which the beaks are situated), are. 

 two depressed and smooth impressions. One of these 

 (fig. 26, ad) is placed near the mouth at the front end of 

 the shell, and marks the point where the front adductor 

 muscle was attached in the living animal. The other 

 (fig. 26, pd) is placed towards the hinder end of the 

 shell, and indicates where the hinder adductor was 

 attached. Running from the one of these impressions 

 to the other is a well-marked line (fig. 26, j^l), which 

 takes a course a little within the margin of the valve, and 

 has a deep indentation (ps) opposite to the hinder end of 

 the shell. This line marks the place where the mantle 

 was attached to the shell, and the indentation or bay 

 marks the point where the muscles which pull in the 

 sijjhon were attached to the shell. 



It foUows from the preceding that if merely shown a 

 single valve of the Mi/a, and knowing nothing of the 

 animal, we should be able to state the following points : 

 I. That the broadest end of the sheU was the one 

 where the mouth was situated, because the beak turns to 

 this end, and this half of the shell is the shortest. (This 

 would not always be true of all Bivalves.) 2. That the 

 animal possessed two adductor muscles for closing the 

 shell. 3. That the animal had breathing-tubes or siphons 

 for conducting the water to the gills, and that these tubes 

 could be partially withdrawn within the shell. 



Ml/a arenaria is found at various points along the 

 British coast, imbedded in sand or mud, generally on 

 long stretches of nearly level shore which are only un- 

 covered at spring-tides. When the tide is out, the posi- 

 tion of the shell is indicated by a rounded or oval hole, 

 from which the animal squirts out water when the foot is 

 put down near it. The shell is situated about five or six 

 inches below the surface, with the siphons pointed up- 



