of teeth, are wider apart on the dorsal half oi' tlie ante- 

 rior border than on the ventral. In this way, an anfi;le 

 is fcirmed, v/hich soon (fig. IG) becomes a ri.";ht anp;le. 

 Meanwhile, the posterior border has grown rapidly and 

 flares outwards so as to give better purchase for the pos- 

 terior adductor during its contraction. Likev/ise, a much 

 smaller portion of the dorsal anterior edge flares out- 

 wards for the attacliment of the anterior adductor (fig. 18). 



There is little modification in form or structure of 

 the shell after the stage siiown in fig. 16, v/liich is a 

 side-viev: of theleft valve from a specimen 5 mm. long. As 

 groY/th on the ventral edge takes place, the knob is con- 

 stantly being added to towards the midline and absorbed on 

 the side tov/ards the concavity of the valve. And as j^rov/th 

 at the posterior border takes place, the posterior adduc- 

 tor is constantly moving backwards. In the larval and 

 subsequent stages, the whole shell, including the teeth, 

 is covered externally by an epidermis. 



The Palettes,- These stinictures are peculiar to the 

 "Ship-v/orms" and have been acquired for the purpose of 

 closing the outer ends of the burrow against intruders. 

 The structure of one of these is shown in fig. 20, which 



