These are very nice...some what lacking in condition but given the rarity of the knives, hilt material and the design of the sheath, I think they could be late 18th century. 
 
The hilts are shell, however, they are not tridacna. 
Tridacna are giant clam shells, these clearly have the spiralling rings of a conch shell. 
 
Gavin
		 
		
		
		
		
		
		
		
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