The post below was the result of a quiz of a fossil I put on GYBO quite a while back. Prem Subrahmanyam was kind enough to not only correct the identification but to also provide the reason (a very interesting one at that). I refer you to his comment below.
----------------------------------Keith nailed this one. It is Lepidodendron. A genus of vascular "trees" from the Carboniferous period (~300MYBP) that are closely related to quilworts (Isoetes) and club mosses (Lycopodium). This partiular specimen was found by a botanist friend in Arkansas. During the Carboniferous period, modern land masses were located along Earth's equator and the world was warmer and more humid. Plants like Lepidodendron lived in the lush swamps that formed the massive coal, oil and natural gas reserves that presently fuel our modern world. As we all know, by re-releasing the carbon of the Carboniferous we are warming the earth and thus altering the patterns of biotic distribution.