Why can't I use Texture2D?

I'm trying to use a Texture2D with UnityGUI to create a button with an icon. However, Unity says that it can't find the namespace for Texture2D. I looked up the namespace and it says it is UnityEngine which I am using I think:

 using UnityEngine;

So what am I missing here? How do I use Texture2D? Here is my code:

 using UnityEngine;
 using System.Collections;
 
 public class ButtonScript : MonoBehaviour {
     public Texture2d icon;
     // Use this for initialization
     void Start () {
     
     }
 
     void OnGUI()
     {
         if (GUI.Button (new Rect (10, 10, 10, 10), icon))
                         Application.LoadLevel ("LevelSelectScreen");
     }
     
     // Update is called once per frame
     void Update () {
     }
 
 }