The genus _Agkistrodon_ includes about nine species in the Old World and three in North and Central America. Some of the primitive characters of the genus are: head covered with nine enlarged shields or having the internasals and prefrontals broken up into small scales; subcaudals on proximal part of tail undivided; fangs relatively short; tail lacking rattles. In one species, _A. rhodostoma_, the scales are smooth; and the female is oviparous and guards her eggs until they hatch. Other species have keeled scales and are ovo-viviparous. There is little paleontological evidence illustrating evolution of the cottonmouth or for that matter of crotalids in general. Brattstrom (1954) summarized the current knowledge of fossil pit-vipers in North America. The few fossils found of the cottonmouth are from Alacha, Brevard, Citrus, Levy, Pasco, and Pinellas counties, Florida (Brattstrom, _op. cit._:35; Auffenberg, 1963:202). All are of late Pleistocene Age and well within the present geographic range of the cottonmouth. Of crotalid genera only _Agkistrodon_ occurs in both the Old World and the New World, suggesting that this genus is relatively old. Schmidt (1946: 149-150) mentioned several other closely related groups of animals found in both eastern Asia and eastern North America, including the reptilian genera: _Natrix_, _Opheodrys_, _Elaphe_, _Ophisaurus_, _Leiolopisma_ (= _Lygosoma_), _Eumeces_, _Clemmys_, _Emmys_, and _Alligator_. Of the groups of animals now confined to these two regions the most important are the cryptobranchid salamanders, the genus _Alligator_, and the spoon-bills (_Psephurus_ in China and _Polyodon_ in the Mississippi drainage). Fossil evidence for these groups indicates that existing forms common to eastern Asia and eastern North America are remnants of a late Cretaceous or early Tertiary Holarctic fauna which was forced southward as the climate became gradually cooler to the north. "Other clues suggest that both _Agkistrodon_ and _Trimeresurus_ (_Bothrops_) moved from Asia to America, one of these presumably giving rise to the rattlesnakes." (Darlington, 1957:228).