A Mesiodens is the most common supernumary (extra) tooth. It is located in the maxilla in the midline (hence the special name which means "middle tooth"). They can be well formed or just little clumps of tooth like stuff. A related phenomenon is the odontoma. They are often discovered on a routine x-ray or perhaps if the teeth are not erupting in a normal manner. Most of the ones I have seen kind of look like teeth with a small crown and a root of some kind. If they form right side up like a regular tooth and are fairly well formed, they may erupt like normal teeth. If it is upside down it definately will not erupt normally. (The second radiograph seen here shows two inverted mesiodens in a young child. Can you see them?) Mesiodens can interfere with the normal eruption of the teeth leading to crowding or impaction. There can actually be more than one up there. I have even seen some teenagers with this funny looking pointed tooth coming in on the palate behind the incisors.