hmmm. Both arugula and watercress are peppery tasting, but I don't think either are purple on the tips or bumpy on the underside.
Kale does sometimes have a purple tip but I'm not sure if it spicy. If it was included in a salad mix, it was probably baby kale (regular kale is generally cooked). Baby red swiss chard might be another option, but again, I'm not sure if it would be spicy.
Here is a link to a page that describes different types of salad greens with some illustrations: http://www.foodsubs.com/Greensld.html
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