Two families of venomous snakes are indigenous to (continental) North America, Crotalinae, the pit vipers and 2) Elapidae.  The only representatives of the elapids, naturally occuring within the United States are of the genera Micrurus, and Micruroides.  These are commonly and collectively known as "Coral snakes". 

These snakes are all similar in appearance. The most distinct difference is noted following the second "ring" from the black "nose" of both snakes. In the Micrurus this "ring" is a broader black band and the Micruroides possess a broad red band.  Following this third ring or band they both follow a pattern with the red contacting the yellow or lighter color. There is also a distinctive difference in color shades. The yellow band of the Micruriodes is more pale, as an "off white". There are no distinct differences that would make snakes of either genus difficult to distinguish as a "coral snake".

The distribution of the Micrurus encompass most of the Southeastern United
States from North Carolina, then south and west into Texas. The Micruriodes
range is limited to further west into Arizona, New Mexico and south into
Mexico.

The Crotalinae found in North America is composed of three genera. These are Crotalus, Sisturus and Akistrodon. Crotalus and Sistrurus are the two genera that the snakes known as rattlesnakes belong to. The former, being the more significant of the two. The more dangerous of the rattlesnakes belong to this group. The latter are smaller snakes and are much less dangerous to humans. Although any venomous snake should be considered dangerous, bites sustained from rattlesnakes of the genus Sistrurus are seldom life threatening. The third genus of North American Crotalinae is Agkistrodon. The moccasins and copperheads belong to this group. Bites that are sustained from species belonging to this genus are serious and are capable of extensive local tissue damage, but fatalities are still rare. 

All the rattlesnakes belonging to the genus Crotalus, excluding one exception, are capable of causing serious envenomation in humans. Despite it’s threatening name the Tiger rattlesnake (Crotalus tigris) cause little local reaction and no significant systemic symptoms in humans. *1 

The author of this document must emphasize that all venomous bites should be considered potentially dangerous!  Individual human immune reactions vary considerably! 

It has long been recognized that snake venoms cannot accurately be classified as being exclusively hematoxic or neurotoxic. All venoms are composed of at least several toxins and are capable of various and numerous effects in living tissue and systems. For the purpose of general considerations, the venoms belonging to the genera of the crotalinae, with few exceptions can primarily be considered hematoxic, hemolytic and cytolytic. 

The most prominent exceptions are, the Mojave rattlesnake (Crotalus scutulatus) and species of the South American rattlesnakes (Crotalus terrificus).  The venom of these two are somewhat similar.  Crotalus scutulatus present an interesting case.   Within the same species, three venom types are recognized.  Type (A) being primarily neurotoxic and type (B) primarily hemotoxic.  There also exist a type (A, B) venom in the same species.  The venom variation appears to be determined by geographical variance.  A similar variation also exists among species of the South American rattlesnakes of the species Terrificus, however there are subspecies that are recognized along with these variations.

It is interesting to note that the venoms containing primary neurotoxic properties are generally considered, by most references, to be considerable more leathel, such as Type (A) and (A, B) Mojavetoxin than type B.  The same comparison can be made between Crotalus terrificus terrificus and the less deadly hematoxic subspecies. *2 

The family Crotalinae are also known as pit vipers. This term refers to the heat sensing facial pits located on each side of the head. The pit lies below the line from the nostril to the eye and below the two. These pits serve as extremely sensitive thermo receptive organs. They aid the snakes in locating their pray. 

The coral snakes belong to the family Elapidae. Their venom is primarily neurotoxic, affecting the Respiratory System. Causing respiratory paralysis. The effects are similar to those of most other elapids, such as the cobras. This comparison is general and there exist substantial difference among the elapid venoms. The Mambas and the Kraits are the other well-known snakes that belong to this family. 

There are two other families of land snakes, Viperinae and Colubridae that should be mentioned. Viperinae refers to the vipers and are all venomous. The venoms of these snakes are mostly hemotoxic with some similarities to the venoms of the Crotalinae species. There are no snakes of this family in the Western Hemisphere. Snakes belonging to the family Colubridae are mostly non-venomous. Most non-venomous snakes belong to this group.  There, however, exist a few species of venomous snakes in this family.  The only significant species worth mentioning here is the Boomslang (Dispholidus typus) and Twig Snake (Thelotorinis capensis).  These are the only snakes of this group known to cause human fatalities.  The Montpellier snake is another that has also been noted as being capable of severe envenomation.  The distribution of venomous snakes belonging to the family colubridae are limited to Africa and in the case of the Montpellier snake ( Malpolon monspessulanus) the Mediteranean coast and up into Atlas Mountains.

 The “sea snakes” are classified in there own group. Their family name is Hydrophiidae. The venom of these snakes is very similar to that of the Elapids.  They are only mostly located in the Indo-Pacific and surrounding tropical seas.  The only sea snake found in the waters of the North, Central and South America is the Yellow –bellied Sea snake (Pelamis Platurus).  It is only found in the Pacific waters along the southwestern coast, most notably along the coast of California, reaching to Hawaiian Islands and south to the Galapagos Islands and Ecuador.

*1 Venomous Reptiles of North America Carl H. Ernst 1992

*2  Critical Care Clinics V15 (2) April 144