Where did the saying happy as a clam come from?

Factual Questions
1

How did the saying “happpy as a clam” come about and what exactly does it mean?

2

Here is the opinion of one site

Wordorigins.org:

http://www.wordorigins.org/wordorh.htm

3

The Word Detective agrees with Markxxx

http://www.word-detective.com/072999.html#clam

4

I had always assumed it was because a clam looks like it’s smiling when it’s shell is closed.