Rat snake
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Rat snakes are a large, polyphyletic, group of snakes from the Colubrid subfamily Colubrinae. There is considerable variation between different types of rat snake but most are medium to large, rodent eating snakes.
Previously most were assigned to the genus Elaphe but many have been since renamed. The validity of some genera is debatable but for the purpose of this article a more liberal taxonomic stance will be taken. Rat snakes have traditionally been divided into two groups, New World and Old World species.
New World Rat Snakes
New World rat snakes belong to the Colubrine tribe Lampropeltinae, and as such are closely related to Lampropeltis (milk snakes and king snakes), Pituophis (gopher snakes, pine snakes and bull snakes (http://en.wikipedia.org/wiki/Bull_Snake)), Rhinocheilus (longnose snakes), Arizona (glossy snakes) and Stilosoma (short-tailed snakes). The entire Lampropeltinid group is descended from Old World rat snakes that crossed the Bering Land Bridge sometime within the last twenty to thirty million years.
The New World rat snakes consist of the genera Bogertophis (Trans Pecos and Baja rat snakes), Pantherophis (Corn snakes, Fox snakes and American rat snakes), Psuedelaphe (Central American rat snake) and Senticola (Green rat snake).
Good Pictures at:
http://www.kentuckysnakes.org/snake_details.cfm?snake_number=33&option=non-posionous
Old World Rat Snakes
The Genera Elaphe, Euprepiophis, Oreophis, Orthriophis, Rhinechis, and Zamenis constitute Old-World rat snakes.
Additional Note
It is worth noting that, though the polyphyletic nature of the genus is almost undisputed, many species are conservatively referred to as Elaphe sp.