Varieties of St Johns Wort

I would like to know how many different varieties of St Johns Wort there are and where can I find pictures of them? The plant I have is very low growing but the leaves and flowers look like Hypericum.

The species of St. John's Wort that is most commonly used in herbal medicine is Hypericum perforatum. There are several varieties of this species and Richters currently carries five (see the online catalog). These are all grow upright, reaching 60-75cm (2 to 2.5 feet) at maturity, although they may begin in their first year of growth as a low, alomost creeping plant.
Your plant could be Hypericum perforatum, or it could be one of dozens of other species of Hypericum. If your plant is a wild plant, there are at least a dozen native American species. The differences between the various species and varieties are subtle and the only sure way to know for sure what you have is to have a trained specialist look at a flowering specimen. Often your local university or extension specialist can help.
The Missouri Botanic Garden has images of a few of the native American species online which may help narrow down the identity of your plant. The link for the plant family Clusiaceae of which Hypericum is a member is mobot.mobot.org/W3T/Search/image/iix187.html.