101 



cumstance of its having the posterior slope closed, or 

 nearly so, and the inner margins dentated at that part,and 

 occasionally interlocking with the opposite valve; from 

 which it may be inferred, that the animal inhabiting this 

 shell does not suspend itself by a tendinous byssus: nor 

 does it ever attain so large a size as the Tridacna, seldom 

 exceeding eight or nine inches in length. The ribs are 

 armed with small tubular spines, or imbrications, never 

 resembling arched or vaulted scales. Only one species 

 is known. Shell equivalve, regular, the posterior side 

 heart-shaped and closed, or nearly so, with dentat- 

 ed edges: hinge having two teeth, compressed, une- 

 qual, anterior, and inserted; ligament external and 

 marginal. 



It appears doubtful whether this species attaches it- 

 self by a tendinous byssus or not, although LaMarck 

 seems to consider it impossible. Many genera of shells, 

 known to spin a byssus, presenfno greater passage for it 

 than the Hippopus maculatus, which in almost all the 

 examples the writer has examined is by no means en- 

 tirely closed at its posterior slope, 



Hippopus maculatus. 



