His underparts are like sharp potsherds is literally “under him points of potsherds.” The reference is to the belly of the crocodile, which is covered with scales. Potsherds is the same word used in 2.8, where Job scrapes his skin with these. See there for comments. As Leviathan crawls through the mud, the scales on his belly leave their marks on the ground. This action is compared to that of a threshing sledge, which consists of parallel boards with sharp stones and is dragged over the threshing floor to remove the grain from the seed coverings. For further details see a Bible dictionary. Mire refers to the muddy ground where the animal leaves and enters the water. “Beneath him his belly is like sharp pieces of broken pottery.” Threshing sledge must often be adapted to some local piece of similar equipment, or a descriptive phrase may be used; for example, “When he drags himself through the mud, he leaves marks as if made by a board with spikes driven through it.”

Quoted with permission from Reyburn, Wiliam. A Handbook on Job. (UBS Helps for Translators). New York: UBS, 1992. For this and other handbooks for translators see here .