know?"

"Because I am looking after that case. I am in St. Louis upon that very
business," replied the man, with a twinkle in his eyes.

"Are _you_ Detective Rider?" questioned the young girl, wonderingly, and
trembling with excitement.

Her companion smiled.

"What do you know about Detective Rider?" he inquired. Then, as she
flushed and seemed somewhat embarrassed, he continued: "And who are
_you_, if you please?"

"I am--I am acquainted with Raymond Palmer," Mona answered, evasively;
"he has told me about the robbery and--"

"Ah! yes. I understand," interposed the quick-witted officer, as he
comprehended the situation. "But sit down and tell me the whole story as
briefly as possible, and I can then judge what will be best to do."

He moved a chair forward for her, then sat down himself, where he could
watch her closely, as she talked, and Mona related all that we already
know regarding the two people whom she had seen upon the street-car,
together with all that followed in connection with the discovery of the
rent in the gray cloth dress, the sending for the fragment that Ray had
preserved, and which had fitted so exactly into the tear.

The detective listened with the closest attention, his small, keen eyes
alone betraying the intense interest which her recital excited.

When she had concluded, he drew forth a set of tablets and made notes of
several items, after which he said:

"Now, Miss ---- What shall I call you? Whom shall I ask for at the hotel,
if I should wish to see you again upon this business?"

"Miss Richards. I am traveling with a Mrs. Montague, of New York," Mona
replied.

"Well, then, Miss Richards, you go back to your hotel, and of course
conduct yourself as if you had nothing unusual on your mind; but hold
yourself in readiness to produce that important bit of cloth, if I
should call upon you to do so within the next few hours. By the way,"
he added, with sudden thought, "if you have it with you, I might as well
take a look at it."

Mona took the paper containing it from her purse and gave it to him.

"You are _sure_ this matches the dress?" he asked, examining it closely.
"We don't want to make any awkward mistakes, you know."

"It is identical. I believe that every thread in this piece can be
matched by a corresponding thread in the garment," the fair girl
asserted, so positively that he seemed to be entirely satisfied.

He returned the piece to her and then arose in a brisk, bu