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.
Treatment: Well, if it's an extra tooth, you want to eventually get it out of there. I usually wait till I see if it is interfering with the normal eruption of the permanent incisors, or shows some other sign that there is trouble. If it's not causing an immediate problem, you can wait a while, usually till the child is 6 to 8 years old to deal with it so long as you are monitoring the situation. If it will erupt on it's own, then we can just extract it like a normal tooth (doesn't happen very often). If it does not or you can just tell that it's going nowhere, it needs to be surgical removed, hopefully "timed" correctly. I don't like to do it too soon as I don't want there to be any damage to the developing tooth buds from the surgical procedure. Eventually, you do need to have them removed. Once it is removed it is likely the new teeth will erupt in a fairly normal pattern.