getElementsByClassName on prototype

Hi there,
Completely stuck with a trivial issue i suppose.
I use the prototype library
I have an element and want to get all its child elements with class 'rooms' and do something with each.
What i do is the following code and i get an error el.getElementsByClassName(".rooms").each is not a function
el.getElementsByClassName(".rooms") return an HTMLcollection...How on earth can i manipulate the elements of this collection?

thanks

el.getElementsByClassName('.rooms').each(function(e){
				//do something
			});

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