head. Currently, 22 genera and 151 species are recognized: These are also the only viperids found in the Americas. The groups of snakes represented here include rattlesnakes, lanceheads, and Asian pit vipers. The type genus for this subfamily is Crotalus, of which the type species is the timber rattlesnake, C. horridus. Timber rattlesnake The Timber rattlesnake, (Crotalus horridus), is a species of venomous pit viper endemic to eastern North America. This is the only rattlesnake species in most of the populous northeastern United States and is second only to its cousins to the west, the prairie rattlesnake, as the most northerly distributed venomous snake in North America. No subspecies are currently recognized. Malayan pit viper The Malayan pit viper (Calloselasma rhodostoma) is an Asian species of pit viper that is reputed to be an ill-tempered snake that is quick to strike in defense. This species is one of the main causes of snakebite envenoming in Southeast Asia. However, mortality rate among untreated bite victims is very low (1–10%). Although bites are common, death is very rare. When a victim dies of a bite it is chiefly caused by haemorrhages and secondary infections. Before specific antivenom became available, the mortality rate in hospitalised patients was around 1% (Reid et al. 1967a). In the study of Reid et al. (1963a), of a total of 291 patients with verified C. rhodostoma bites, only 2 patients died, and their deaths could only be indirectly attributed to the snakebites. One patient died of tetanus and one from a combination of an anaphylactic reaction to the antivenom, an intracerebral haemorrhage and severe pre-existing anaemia. In 23 fatalities due to C. rhodostoma bites recorded in northern Malaysia between 1955 and 1960, the average time between the bite and death was 64.6 h (5–240 h), the median time 32 h (Reid et al. 1963a). According to a study of fatal snakebites in rural areas of Thailand, 13 out of 46 were caused by C. rhodostoma (Looareesuwan et al. 1988). The local necrotising effect of the venom is a common cause of morbidity. Gangrene can lead to the loss of toes, fingers or whole extremities; chronic infections (osteomyelitis) can also occur. The intravenous for Malayan pit viper venom is 6.1 mg/kg mouse and the average venom yield per bite is 40–60 mg (dry weight). Sharp-nosed pit viper The Sharp-nosed pit viper or hundred pacer (Deinagkistrodon acutus) is another Asian