My American Government book says that anarchy and oligarchy both are forms of dictatorship.
I would gander a guess that your American Government book is wrong. How can anarchy be a dictatorship. Anarchy has no governing person or body, so how could it be a dictatorship? A dictatorship is where a person or party has absolute rule, so how in gods name can an anarchy be a dictatorship. You should mention to your teacher that the book he's making you use is crap.