“We have to bear in mind,” remarked Summerlee, “that there are many prehistoric forms which have never come down to us. It would be rash to suppose that we can give a name to all that we are likely to meet.”

“Exactly. A rough classification may be the best that we can attempt. Tomorrow some further evidence may help us to an identification. Meantime we can only renew our interrupted slumbers.”

“But not without a sentinel,” said Lord John, with decision. “We can’t afford to take chances in a country like this. Two-hour spells in the future, for each of us.”

“Then I’ll just finish my pipe in starting the first one,” said Professor Summerlee; and from that time onwards we never trusted ourselves again without a watchman.