The back corner that 'wedges' out is where the maxillary venom glands are, which is why that area usually look more pronounced than in nonvenmous snakes. One thing you'll learn if you study snakes (or really anything) is that there's always exceptions to the rule. It hard to give anything a broad catigorization. Often a rules that works in one country or geographic region doesn't work in another (take coral snakes for example.)