This lifeform is found in the SW USA (Texas, New Mexico, and Arizona). This lifeform is found in Mexico.

Ridge Nosed Rattlesnake (Crotalus willardi) is found in the highlands of Mexico occasionally as far north as the New Mexico/Mexico border. This is a small species, typically less than 20 inches in length.

Rattlesnakes (Genus Crotalus & genus Sistrurus) are a group of snakes found primarily in the southern United States and in Mexico. Most of the species are found in the southwestern United States of Arizona, New Mexico, and adjacent areas. However, one species, the Neotropical Rattlesnake (Crotalus durissus), is found from Mexico to Argentina. These snakes are characterized by having a unique rattle at the end of their tail that is shaken when these animals are aroused. Rattlesnakes are known for the speed of their strike and their very long fangs which are folded back into the mouth when not in use. Their venom can be deadly. Following is a partial species list (key: * = pictured):

Thirty species in the family are listed here. Eighteen of the more common and more widespread species are pictured and discussed. (C. aquilus is subspecies of C. triseriatus.)

     SCIENTIFIC NAME    COMMON NAME          DISTRIBUTION

     C. adamanteus*     E. Diamondback       SE USA
     C. atrox           W. Diamondback       SW USA & nearby Mex.
     C. basiliscus      Mex. West Coast      W. Coastal Mexico
     C. catalinensis    Catalina Rattleless  St. Catalina (Gulf)
     C. cerastes*       Sidewinder           SW USA & nearby Mex.
     C. durissus*       Neotropical          Neotropics
     C. enyo*           Baja                 Baja California
     C. exsul           Cedros Island        Cedros Island
     C. horridus*       Timber               East USA
     C. intermedius     Small Headed         Local S. Mexico
     C. lannomi         Autlan (one known)   Jalisco, W. Mexico
     C. lepidus*        Rock                 SW USA and adj Mex.
     C. mitchellii*     Speckled             SW USA and Baja
     C. molossus*       Black Tail           SW USA and Mexico
     C. polystictus     Lance Headed         SC Mexico
     C. pricei*         Twin Spot            Mexico Highlands
     C. pusillus        Tancitaran Dusky     Local SW  Baja
     C. ruber*          Red Diamond          S. California & Baja
     C. scutelatus*     Mojave               SW USA & Mexico
     C. stejnegeri      Long Tail            Local WC Mexico
     C. tigris*         Tiger                Sonora and adj USA
     C. tortugensis     Tortuga Island       Tortuga in Gulf
     C. transversus     Cross Banded         Local S. Mexico
     C. triseriatus     Mexican Dusky        S. Mexico
     C. tris. aquilius* Queretaran Dusky     SC Mexico
     C. viridis*        Western              West US
     C. willardii*      Ridge Nosed          N.Mex Highlands,etc.
     S. catenatus*      Massassauga          C. USA
     S. miliarius*      Pygmy                SE USA
     S. ravus*          Mexican Pygmy        SC Mexico - Local

Vipers (Family Viperidae) are an almost worldwide group of poisonous snakes with excellent striking ability. They open their mouths very wide when striking and their hollow fangs can penetrate deeply to permit large amounts of venom to be deposited. Any bite should be considered very serious and should get immediate medical attention. Many of the species have very deadly venom. Included in this group are the rattlesnakes, copperhead, and water moccasins of the New World.  Also included in this family are the Old World adders and vipers.  There are about 230 species in the Viperidae family. This family can be further divided into subfamilies.  The two best known sub-families are the Old World Vipers (w\o pits) (Viperinae) and the Pit vipers (Crotalinae).

Lizards and Snakes (Squamata Order) share many common characteristics and consequently they are grouped in a single order. There are greater differences between some groups of lizards than there are between other groups of lizards and snakes. The same is true of snakes. Lizards and snakes share a common skull shape.

Reptiles (Class Reptilia) are an ancient group of scaled  chordates. These scales may be permanently joined, as in the  turtles, or flexible, as in the snakes. Reptiles are land-based. Their eggs are laid on land and the young are air breathing.

Backboned Animals (Phylum Chordata) are the most advanced group of animals on earth. These animals are characterized by having a spinal cord or backbone. Most members have a clearly defined brain that controls the organism through a spinal cord. Fish, amphibians, reptiles, birds, and mammals are in this phylum.

Currently, some taxonomists believe that the fish should be divided into two groups (sharks and regular fishes) and that there are some other primitive groups in the phylum such as hagfish or lampreys.

Animal Kingdom contains numerous organisms that feed on other animals or plants. Included in the animal kingdom are the lower marine invertebrates such as sponges and corals, the jointed legged animals such as insects and spiders, and the backboned animals such as fish, amphibians, reptiles, birds, and mammals.