1a. Legs present……………………………………………………………………......................Suborder Lacertilia
1b. Legs absent……………………………………………………………………………......................................2

2a. Eyelids and ear openings present……………………………….……...............................Ophisaurus attenuatus
2b. Eyelids and ear openings absent……………………………….……...…...............................Suborder Serpentes

1a. Dorsal and ventral scales small, uniform in size; eyes reduced……………...............................Leptotyphlops dulcis
1b. Dorsal and ventral scales distinctly different; eyes not reduced.………………………….......................................2

2a. Pupils round (sometimes oval)………………………………………..…………..…………..............................10
2b. Pupils elliptic (cat-eyed)……….…………………………………………………………….……......................3

3a. Deep facial pit between eyes and nostrils……………………………………………...........................................4
3b. No facial pit between eyes and nostrils..…………….............................................................Hypsiglena torquata

4a. Rattles present……..……………………………………………………………………..……….....................5
4b. Rattles absent……..………………………………………………………………………..…….….................9

5a. Head with nine large cranial scales……………………………………….…………………...............................6
5b. Head with numerous small scales……………………….……………………...…………..................................7

6a. Body gray with spots, sometimes with rusty mid-dorsal stripe…….………................................Sistrurus miliarius
6b. Body brown with “figure 8” blotches; no mid-dorsal stripe……….…………...........................Sistrurus catenatus

7a. Body with V-shaped (chevron) bands; rusty mid-dorsal stripe…....………….............................Crotalus horridus
7b. Body with blotches or diamond-shaped markings; no mid-dorsal stripe……..........................................................8

8a. Body with diamond-shaped markings; alternating black and white bands on tail.....…...…….............Crotalus atrox
8b. Body with “figure 8” markings on brown or green ground color….…………...........…...................Crotalus viridis

9a. Body dark, almost patternless, and robust; habitat near water………..................................Agkistrodon piscivorus
9b. Body color variable, light brown to bronze; hourglass bands across back….........................Agkistrodon contortrix

10a. Scales smooth or weakly keeled...……………………………………........…………….………...................11
10b. Scales keeled (raised ridge in center of scale)…………………………........................………………............25

11a. Anal scale single……………………………………………………......………………….............................12
11b. Anal scale divided…………………………………………………….............…………...............................17

12a. Belly checkered with brown or black pigment………………………….....……………..................................13
12b. Belly unmarked or with little pigmentation…………………………….......…………………...........................15

13a. Body black with white or yellow spots…………….……...……….....….............................Lampropeltis getula
13b. Body not as above…………………………………………………........…………………….......................14

14a. Body with red or brown blotches or spots outlined in black……..….......…...................Lampropeltis calligaster
14b. Body with alternating bands of red, black, and yellow or white……..............................Lampropeltis triangulum

15a. Body with brown blotches……………………………………….......…………...................…Arizona elegans
15b. Body not as above……………………………………………………………….......................................…16

16a. Red saddles on back bordered by black……………………………….............................Cemophora coccinea
16b. Red and black crossbands; subcaudals in single row…………………...............................Rhinocheilus lecontei

17a. Belly orange, red, or pink ………………………………………………………………............................…18
17b. Belly not as above…………………………………………………………..………….....….....................…20

18a. Orange or red ring around neck, may or may not complete…..………….............................Diadophis punctatus
18b. Neck ring absent…..…………………………………………………………..…................…..................…19

19a. Dorsum black with red and black belly………………….…….…………..............................Farancia abacura
19b. Dorsum purplish black with salmon pink belly……..…...……………....................................Carphophis vermis

20a. Nose pointed; eyes large and forward……………………………………………......……….........................21
20b. Nose blunt; eyes lateral..…………………………………………………………...…....................................22

21a. Scale rows 13 in front of tail…………………………………………….............................Masticophis flagellum
21b. Scale rows 15 in front of tail……………………………………...…….................................Coluber constrictor

22a. Head darker than rest of body (bicolored)………………………………………………..........................……23
22b. Head color typically same as body (concolored).…………………………............................................………24

23a. Head color faded; posterior end of cap is concaved……..…….……..…….............................…Tantilla gracilis
23b. Head color black; posterior end of cap is convexed……..…….…….....................…….......…Tantilla nigriceps

24a. Weakly keeled scales; body peppered or plain; loreal scale horizontal and touching eye; 6 upper labial
scales....................................................................…………..…………......................................…Virginia valeriae
24b. Smooth scales; body variable, with crossbands, collar, or plain; loreal scale not touching eye; 7 upper labial
scales...........................................................................................……...…................................Sonora semiannulata

25a. Anal single…………………………………………………………………………........................…………26
25b. Anal divided……………………………………………………………………….............................………33

26a. Body with blotches; four prefrontal scales on crown…….……………...................................Pituophis catenifer
26b. Body with longitudinal lines……………………………………………………………….......................……27

27a. Belly with parallel rows of spots………………………………….................................Tropidiclonion lineatum
27b. Belly plain…………………………………………………………………................................……………28

28a. Light line on scale rows 2 and 3…………………………………………………............................…………29
28b. Light line on scale rows 3 and 4…………………………………………….………...............................……32

29a. Large black blotch on each side of neck.....……………..……………………..................Thamnophis cyrtopsis
29b. Black blotch absent………………………………………………………………....………..................……30

30a. Black squares “checkering” sides………………….……………................................…Thamnophis marcianus
30b. Sides peppered with spots…………………………………………………................................……………31

31a. Location: NW Panhandle (Black Mesa)………………………………….......................…Thamnophis elegans
31b. Not as above………………………….…………………………………….......................Thamnophis sirtalis

32a. Alternating row of black spots between stripes………..…………………...........................…Thamnophis radix
32b. Back uniformly black between stripes…………….….…….…........................................Thamnophis proximus

33a. Rostral scale turned up, nose pointed………………………………………………....…….....................……34
33b. Rostral scale not turned up, nose rounded………………………………………….....……….....................…35

34a. Belly plain……………………………………..………….…..……………....................Heterodon platirhinos
34b. Belly with black ventral scales…………………..……….………..………............................Heterodon nasicus

35a. Body blotched or banded…………………………………………………………….....................................36
35b. Body plain or with longitudinal stripes……………………………………………………....……...................41

36a. Squared ventral scales……………………………………………………………………..............................37
36b. Ventral scales without squared edges…………………………………………………….…….......................38

37a. Mostly black (adults), may have yellow or red accents; head lacking "spearpoint"; juveniles with post-ocular stripe
  stopping at corner of mouth; underside of tail spotted or plain….................................................Scotophis obsoletus
37b. Gray body color; red blotches outlined in black; "spearpoint" on head; juveniles with post-ocular stripe continuing
  beyond corner of mouth; underside of tail striped......................................................................Pantherophis emoryi

38a. Body banded, in whole or in part………………………………………......……………………....................39
38b. Body with blotches only………………………………………………..........……………………….............40

39a. Body with bands only…………………………………………………........………................Nerodia fasciata
39b. Body with bands posterior and blotches anterior……………………….....…….......................Nerodia sipedon

40a. Dark blotches with light crossbars………………..……………...……...…....................Nerodia erythrogaster
40b. Light blotches with dark crossbars………………...……………………............…..............Nerodia rhombifer

41a. Body uniform in color; belly plain…………………...…..………………........……………………….............42
41b. Body plain or with longitudinal stripe(s)…………....…………………….........…………………....................43

42a. Body long and slender; plain green above with bright yellow belly……..................................Opheodrys aestivus
42b. Body plain brown; head small and pointed…………………………….....…...........................Virginia striatula

43a. Light mid-dorsal line (often zigzag).between parallel rows of spots……….………………................................44
43b. Back plain…….……………………………………………………….....……………….............................45

44a. Belly plain brown……………………………………………………….....….............................Storeria dekayi
44b. Belly plain red……………………………………………………….........................Storeria occipitomaculata

45a. Broad yellowish stripe on scale rows 1, 2, and 3……………………......……...........................Regina grahamii
45b. Lateral stripe faint or absent………………………………………......……………......................Regina rigida