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Dichotomous Key to Oklahoma Snakes

**This dichotomous key is a general guide for the snakes of Oklahoma.  Currently, an official key to Oklahoma's reptiles does not exist.  In order to key out and identify Oklahoma's reptiles, one must use numerous keys from surrounding states and put together a list of possibilities.  This key has been compiled and written by Richard Butler, using the Peterson Field Guide to Reptiles and Amphibians of the Eastern and Central United States, Canada, and Mexico.  This is one of the best field guides, but of course, there are many others that may work just fine.**

This key, for now, is only for snakes.  I am working to include lizards and amphibians.  I have made this key to be relatively simple, beginning with the most basic and noticeable features.  Through the process of elimination, one should be able to identify any snake captured from anywhere within the state, and some from other states.  However, this key should only be used for the sole purpose of identifying those species found within the state of Oklahoma.

Order Squamata
(Snakes and Lizards)

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

Suborder Lacertilia - Lizards
(Under Construction)

Suborder Serpentes - Snakes


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…...........................................Pantherophis 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


Last Modified June 20, 2008