The magazine he wanted had been put away under a pile of papers, and as the boy was getting it out Flossie caught sight, down the platform, of a man pasting up on the advertising boards in the underground station, some new posters.

“Oh, maybe it’s signs about a circus, Freddie!” cried the little girl “Come on and watch!”

Freddie was always ready to go, and he had darted off after his sister down the long platform before Bert and Nan saw them.  When the two older children missed the younger twins they looked hurriedly about for them.

“There they are—­watching that bill-poster,” said Bert.  For the underground subway stations are much used by advertisers, gaily colored sheets of paper being pasted on boards put there for that purpose.

“You mustn’t run away like that!” said Nan to Flossie, as she came up to her sister, to lead her back.

“We wanted to see if it was a circus poster, but it isn’t,” returned Freddie.

“Well, come on back.  Daddy will miss us,” declared Bert.  He started back—­at least he thought he did—­for the place where their father had told them to wait for him.  But the subway station under the New York sidewalks was so large and rambling, there were so many stairways leading here and there, up and down, and there were so many platforms that it is no wonder Bert went astray.

“Where are you going?” asked Nan at last.

“Well, I was trying to find the place father told us to wait,” Bert answered.

“It’s over this way,” said Nan, pointing just the other direction from the one in which Bert was walking.

“All right, we’ll try that, but it seems wrong,” he stated.

They walked a little way in that direction.  They saw nothing of their father, however, and there were fewer people on the platform where they now were.

“Oh, dear!” cried Flossie, “I’m thirsty!  I want a drink!”

“So do I!” added Freddie.

Nan and Bert looked about them.  They were still in the underground station, and they could see trains coming in and going out, and crowds of people hurrying to and fro.  But they could not see their father nor the place where he had told them to wait.  At last Nan said: 

“Bert, I don’t know where we are!  We’re lost!”

CHAPTER XI

FREDDIE AND THE TURTLE

Bert Bobbsey looked all around the big underground subway station before he answered Nan.  Then he took off his cap to scratch his head, as he often did while thinking.  Next he looked down at Flossie and Freddie.

If he thought he was going to find the two little twins in a fright at what Nan had said about being lost, Bert was mistaken.  The two flaxen-haired tots were looking down the long platform, into the gloom of the long tunnel of the subway.

“Aren’t they funny, Freddie?” asked Flossie.

“Yep, awfully funny,” was Freddie’s answer.