What is an iron spider that Caroline was cooking in on Little House on the Prairie?

In the Little House books when Laura and her family were travelling by covered wagon, Laura makes many comments about Ma cooking meals on an iron spider.  She cooked things like fat salt pork, prairie hen and other meals as if it was a regular cast iron frypan.

What makes a spider different from a frying pan is that it has cast iron legs on the bottom of it, so that it is more stable when cooking over an open fire, which is what Caroline did each night and morning while they were travelling westward.

Spiders are fairly uncommon today, although you can still find them in antique shops or on eBay.