for her. His reflections took a
new form.

What of Gaston Max, foremost criminologist in Europe, who now lay dead
and mutilated in an East-End mortuary? The telephone message which had
summoned Dunbar away had been too opportune to be regarded as a mere
coincidence. Mlle. Dorian was, therefore, an accomplice of a murderer.

Stuart sighed. He would have given much--more than he was prepared to
admit to himself--to have known her to be guiltless.

The identity of the missing cabman now engaged his mind. It was quite
possible, of course, that the man had actually found the envelope in
his cab and was in no other way concerned in the matter. But how had
Mlle. Dorian, or the person instructing her, traced the envelope to
his study? And why, if they could establish a claim to it, had they
preferred to attempt to steal it? Finally, why all this disturbance
about a blank pieced of cardboard?

A mental picture of the envelope arose before him, the number, 30,
written upon it and the two black seals securing the lapels. He paused
again in his walk. His reflections had led him to a second definite
point and he fumbled in his waistcoat pocket for a time, seeking a
certain brass coin about the size of a halfpenny, having a square
hole in the middle and peculiar characters engraved around the
square, one on each of the four sides.

He failed to find the coin in his pocket, however, but he walked
briskly up a side street until he came to the entrance to a tube
station. Entering a public telephone call-box, he asked for the
number, City 400. Being put through and having deposited the necessary
fee in the box:

"Is that the Commissioner's Office, New Scotland Yard?" he asked.
"Yes! My name is Dr. Keppel Stuart. If Inspector Dunbar is there,
would you kindly allow me to speak to him."

There was a short interval, then:

"Hullo!" came--"is that Dr. Stuart?"

"Yes. That you, Inspector? I have just remembered something which I
should have observed in the first place if I had been really wide-awake.
The envelope--you know the one I mean?--the one bearing the number,
30, has been sealed with a Chinese coin, known as _cash._ I have just
recognized the fact and thought it wise to let you know at once."

"Are you sure?" asked Dunbar.

"Certain. If you care to call at my place later to-day I can show you
some _cash._ Bring the envelope with you and you will see that the
coins correspond to the impression in the wax. The inscriptions