$(function(){
  $("li:first-child").addClass("first_item");
  $("li:last-child").addClass("last_item");
});

Since :first-child and :last-child aren't compatible with all modern browsers I thought I'd post this snippet.  It make automating the class creation easier too instead of having to remember when typing up your HTML or creating logic in your programming.

Rename "first-item" and/or "last-item" to whatever class you want to use.  You can change the "li" part of the code to whatever element your want or even remove it altogether.

jQuery - Add First and Last Class to LI

css, jquery

jQuery