Frank had agreed to this; and when Mark went to Tallahassee he bought six of the best steel traps he could find. These had been carefully set in likely places along the river, baited with fresh fish, and visited regularly by one or the other of the boys twice a day. At first they had been very successful, as was shown by the ten fine otter-skins carefully stretched over small boards cut for the purpose, and drying in the workshop; but then, their good fortune seemed to desert them. As the season advanced, and the weather grew warmer, they began frequently to find their traps sprung, but empty, or containing only the foot of an otter. At first they thought the captives had gnawed off their own feet in order to escape; but when, only the day before the one with which this chapter opens, they had found in one of the traps the head of an otter minus its body, this theory had to be abandoned. "I never heard of an otter's gnawing off his own head," said Frank, as he examined the grinning trophy he had just taken from the trap, "and I don't believe he could do it anyhow. I don't think he could pull it off either; besides, it's a clean cut; it doesn't look as if it had been pulled off." "No," said Mark, gravely; for both boys had visited the traps on this occasion. "I don't suppose he could have gnawed off, or pulled off, his own head. He must have taken his jack-knife from his pocket, quietly opened it, deliberately cut off his head, and calmly walked away." "I have it!" exclaimed Frank, after a few minutes of profound thought, as the boys paddled homeward. "What?" asked Mark--"the otter?" "No, but I know who stole him. It's one of the very fellows that tried to get me." "Alligators!" shouted Mark. "Yes, alligators; I expect they're the very thieves who have been robbing our traps." The next day at noon, when Mark finished his work at the mill, he hurried back to the ferry to see what Frank meant when he called him that morning, and said he had something to tell him. Frank had gone to the other side of the river with a passenger, but he soon returned. "Well, what is it?" asked Mark, as he helped make the boat fast. "It's this," said Frank. "I've seen a good many alligators in the river lately, and I've had my eye on one big old fellow in particular. He spends most of his time in that little cove down there; but I've noticed that whenever a dog barks, close to the river or when he is crossing on the ferry, the old 'gator paddles out a little way from the cove, and looks very wishfully in that direction. I know alligators are more fond of dog-meat than anything else, but they won't refuse fish when nothing better offers. Now look here."