Snakes of the genus Agkistrodon are relatively primitive members of the Crotalidae, which is one of the most specialized families of snakes. A majority of the pit-vipers are found in the Americas, but close relatives are found from extreme southeastern Europe through temperate Asia to Japan (A. halys) and southeastern Asia including Indonesia (Agkistrodon and Trimeresurus). Familial characters include: vertical pupil of the eye; facial pit present between the preoculars and loreal; scales usually keeled; short, rotatable maxilla bearing a large hollow fang; toothless premaxilla; chiefly hematoxic venom; and undivided anal plate.

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).

The named, American kinds of Agkistrodon currently are arranged as three species: the copperhead, the cantil and the cottonmouth. The copperhead (A. contortrix) is divided into four subspecies, all of which are terrestrial. This species occurs from southern New England to eastern Kansas and along the Atlantic and Gulf Coastal plains, exclusive of peninsular Florida and the delta of the Mississippi River in Louisiana. It extends southwest from Kansas through the Edwards Plateau of west-central Texas. Isolated populations occur in the Chisos and Davis mountains of Trans-Pecos Texas. The cantil or Mexican moccasin (A. bilineatus), probably the nearest relative of the cottonmouth (A. piscivorus), is divisible into two subspecies and occupies a nearly complementary range from Mexico south to Nicaragua. The cottonmouth occurs throughout the coastal plains of the southeastern United States, usually at altitudes of 500 feet or less. Two subspecies are recognized, the eastern A. p. piscivorus and the western A. p. leucostoma. A revision of the genus is underway by Professor Howard K. Gloyd.

The basic pattern and various behavioral traits are common to all three species. The young are more nearly alike in appearance than adults, the copperhead and the cottonmouth being easily confused. Adults differ in color, size, body proportions, habitat, and habits. In range and habitat preference the cottonmouth more closely resembles the southern subspecies of the copperhead, A. c. contortrix, which is usually found in lowlands, near swamps and streams, but seldom in water.

Fig. 1. Geographic range of the cottonmouth, showing marginal and near-marginal records, based largely upon maps by Gloyd and Conant (1943:165) and Conant (1958:336) but including additional records. The more important of these records (from east to west) are discussed in the following paragraphs. Crosshatching indicates the area of intergradation between the eastern and western subspecies. Old records, indicated by dates, and their sources are as follows: 1850's and 1891—U. S. National Museum numbers 4263 and 32753 respectively; 1897—Hurter (1897); and 1895—Stejneger (1895:408).

The northernmost record for the eastern subspecies is in the Petersburg area, Prince George County, Virginia (Anon., 1953:24). A sight record (Hickman, 1922:39) near Bristol, West Virginia, probably was based on a water-snake (Natrix sp.), since the stream in which the snake was seen flows north into the Ohio River rather than southeast through Virginia. In North Carolina the most inland record is from the Neuse River, six to eight miles east of Raleigh (Stejneger, 1895:408). Neill (1947:205) reported a population in the vicinity of Dry Fork Creek on the boundary line of Wilkes and Oglethorpe counties, Georgia. Distribution of cottonmouths in Florida is statewide, including the Keys and other offshore islands.

The ranges of the two subspecies, piscivorus and leucostoma, meet near the eastern border of Mississippi. A. p. piscivorus has been reported from Tishomingo County to the Gulf and east of the Loess Bluff area in central Mississippi, and A. p. leucostoma has been reported from this area westward. A few specimens from along the Coast indicate intergradation (Cook, 1962:33) between the two subspecies.

Barbour (1956:33) reported one specimen from Cypress Creek, in the Green River drainage, Muhlenberg County, Kentucky, and stated that suitable habitat can be found in several areas east of Kentucky Lake. Hence, cottonmouths may have entered this area via the Ohio River. Stejneger (loc. cit.) reported the species in the Wabash River at Mount Carmel, Wabash County, Illinois, and mentioned a former occurrence at Vincennes, Knox County, Indiana; but there are no recent records at these localities. Hurter (1897) reported having seen cottonmouths in Illinois, opposite St. Louis; Smith (1961:265) believes that this and a population in Monroe County, Illinois, are isolated relicts, since no specimens have been found within 50 miles to the south of Monroe County. The specimens reported by Anderson (1941:178; 1945:274) near Chillicothe (three miles southwest and seven miles northwest, respectively), Livingston County, Missouri, also are thought to represent a relict population. Hall and Smith (1947:453) reported one specimen from Jasper County, Missouri, in the Spring River which flows through extreme southeastern Kansas and into Oklahoma and another in the Neosho River at Chetopa, Kansas. Both of these specimens were taken after a flood, and no additional specimens have been taken in this region. Nevertheless, sufficient habitat is probably available along the Neosho and Verdigris rivers in the southeastern part of Kansas.

In Texas the cottonmouth has penetrated marginal habitat perhaps farther than anywhere else in its range. Formerly it was thought to be limited to the country east of the Balcones Escarpment (Smith and Buechner, 1947:8), but semiarid areas of the state have been invaded primarily via the Colorado and Brazos River systems up to altitudes of 2300 feet. Two additional specimens are said to have been collected along the Rio Grande. Dr. Howard K. Gloyd (in litt.) stated that the specimen reported from Eagle Pass, Maverick County, is believed to have been taken in the 1850's; and the one said to have come from the mouth of the Devil's River is actually marked "near Santa Rosa, Cameron County, September 30, 1891." No additional specimens have been taken in that area; and the range now probably extends no farther south than Corpus Christi, Texas. Brown's (1903:554) knowledge of the extension of the range of the cottonmouth west of longitude 98° is probably based upon the records along the Rio Grande reported in the nineteenth century.

Three extensions of the known range in Texas are reported herein. One specimen was captured by Mr. Harry Green (HWG 346) along the San Saba River, 8.1 miles west of Menard, Menard County. The other two specimens (KU 84375 and 84376) were taken by the late Paul Anderson one and one-half miles north of Pecan Crossing, South Concho River, Tom Green County, and one mile west of Mertzon, Irion County.

In the hypsithermal period following Pleistocene glaciation, cottonmouths gradually moved northward occupying areas beyond their present range. The distributional records since the 1850's and the apparent relict populations now in existence indicate that the range of this species has since receded.