A colon is used in sentences where the author wishes to state the order of things. A semi colon which looks like the colon is used to set apart two independent sentences. In the sentence above, a colon is the right punctuation to use because the author states the order of important points.
Before he begins stating this order, he attaches a colon to show that a list is being made. Colons are used to distinguish lists, examples and quotes.
Walter Cunningham's jaw muscle twitches because he is ashamed and self conscious about his low income. The teacher is new to Maycomb and is unaware of this economic difference that is present in Maycomb.
