It seems to me that in the not-too-distant past, there were thought to be three species in the genus Sequoia: Sempivirens (Coast Redwood), Gigantis (Giant Sequoia), and Glyptostrobides (dawn redwood). Wikipedia rather nicely says they are three species of coniferous trees known as redwoods. Apparently, there is still a scientific connection among the three - they are in the subfamily Sequoioideae.