56 
MACTRA. 
outward ligament ; for there are few, if any, in- 
stances, in which it is found without the latter. 
In Mactra, in the pectinated Ostrea, and in Mya, 
there are both ; but the internal tendon seems 
designed to give a firmer connexion and greater 
tenacity to the valves, where, for want of a suffi- 
ciently deep insertion of the teeth, some strong tie 
is requisite to prevent a lateral motion. Thus it 
does not exist in Tellina, Cardium, or Venus. 
To a hieading-trough, we can trace but 
little resemblance in our Mactra. The word was, 
however, no doubt, at first selected for some good 
reason of which we are now^ no judges. 
