nable."

"And you think then," said the doctor, "that yours which you saw were
great serpents swimming on the surface?"

"No, sir, I thought they were something else."

"What?" said Jack, with a certain amount of eagerness.

"They struck me as being those great lizard things which they find
turned into fossils out Swanage and Portland way. I dare say you've
seen specimens of them in the British Museum."

"No," said Jack, colouring a little, "I have never taken any interest in
such things."

"No?" said the captain wonderingly. "Ah, well, perhaps you will. Now
it struck me that these things were--were--Do either of you gentlemen
remember the name of them?"

"Plesiosaurus. Lizard-like," said Sir John.

"That's it, sir," cried the captain, glancing at the speaker, and then
looking again at Jack. "And I tell you how it struck me, and how I
accounted for their being so seldom seen."

"Yes!" said Jack, who had laid down his knife and fork, and was leaning
forward listening attentively. "How did you judge that?"

"From its large eyes."

"What had that to do with it?"

"It meant that it was a deep-sea living creature. You'll find, if you
look into such matters, sir, that things which live in very deep water
generally have very large eyes to collect all the light they can."

"But yours were living on the top of the water," said Jack.

"To be sure," cried the doctor, giving Sir John a sharp glance. "Come,
captain, that's a poser for you."

"Well, no, sir," replied the captain modestly, and with a quiet smile;
"I think I can get over that. Perhaps you know that fish which live in
very deep water, where the pressure is very great, cannot live if by any
chance they are brought to the surface. The air-vessels in them swell
out so that they cannot sink again, and they get suffocated and die."

"But if it was their natural habit to live in deep water," said Jack,
"they would not come to the surface."

"If they could help it, sir," said the captain; "but when a creature of
that kind is ill it may float toward the surface, and turn up as you see
fishes sometimes. I fancy that my great lizard things are still
existing in some places in the mud or bottom of the sea, that they are
never seen unless they are in an unnatural state, and then they soon
die, and get eaten up by the millions of things always on the look-out
for food, and their bones sink."

"I should like to see one," said Jack thoughtfully.

"An