• Presence of external, furlined cheek pouches: 2
• Absence of external, furlined cheek pouches: 15

• Front feet much larger than hind feet; ear (pinna) short and inconspicuous; tail about half the length of head and body (pocket gophers): 3
• Front feet much smaller than hind feet; ear (pinna) conspicuous; tail as long as (or longer than) head and body (pocket mice and kangaroo rats): 5

• Upper incisors not grooved on outer face; claws of front feet relatively small and slender: Thomomys bottae (Botta’s pocket gopher).
• Upper incisors distinctly grooved on outer surface; claws of front feet large and long (longest ones about 15 mm): 4

• Upper incisors with one deep groove; feet blackish: Cratogeomys castanops (yellow-faced pocket gopher).
• Upper incisor with two distinct grooves; feet whitish (species of the genus Geomys):

Seven species of the genus Geomys occur in Texas. These are cryptic species, identifiable primarily on the basis of geographic distribution and characters of the karyotype and genes. Only specialists working with prepared study specimens can identify them using morphological features.

(1) Geomys arenarius (desert pocket gopher) occurs in El Paso and Hudspeth counties in far western Texas.

(2) Geomys attwateri (Attwater’s pocket gopher) occurs in the south-central part of eastern Texas.

(3) Geomys breviceps (Baird’s pocket gopher) occurs in eastern and northeastern Texas.

(4) Geomys bursarius (plains pocket gopher) occurs in northwestern and north-central Texas.

(5) Geomys knoxjonesi (Jones’ pocket gopher) occurs on the southwestern plains of Texas.

(6) Geomys personatus (Texas pocket gopher) occurs in the southern part of Texas.

(7) Geomys texensis (Llano pocket gopher) occurs in the Llano Basin region of the Hill Country in central Texas and in an isolated area on the northern border of the South Texas Plains.

• Hind legs more than twice as long as front legs; tail long and bushy at end; head broad, 25 mm or more in width (kangaroo rats): 6
• Hind legs less than twice as long as front legs; head about 15 mm in width (pocket mice): 10

• Large size, total length of adults 300 mm or more; tip of tail with conspicuous white "banner": 7
• Smaller, total length of adults usually less than 250 mm; tip of tail usually dusky, not white: 8

• Hind foot (from tip of longest claw to heel) 50 mm or more in length; length of tail about 200 mm: Dipodomys spectabilis (banner-tailed kangaroo rat).
• Hind foot less than 50 mm; tail normally less than 200 mm: Dipodomys elator (Texas kangaroo rat).

• Hind foot with five toes (one is very small and difficult to detect): 9
• Hind foot with only four toes:Dipodomys merriami (Merriam’s kangaroo rat).

• Pelage long and silky, brownish; mastoid bullae greatly inflated, giving skull a triangular appearance; interparietal narrow and triangular in shape: Dipodomys ordii (Ord’s kangaroo rat).
• Pelage short and coarse, with orangish cast; mastoid bullae less inflated; interparietal broad and rectangular to roundish in shape: Dipodomys compactus (Gulf Coast kangaroo rat).

• Size small, total length 100 to 130 mm; weight 6 to 8 grams; pelage silky and soft: 11
• Size larger, total length 150 mm or more; pelage harsh, often bristly, never silky: 12

• Length of tail usually 60 mm or more; total length usually 120 mm or more; length of skull usually more than 21 mm; postauricular patch inconspicuous: Perognathus flavescens (plains pocket mouse).
• Length of tail usually less than 60 mm; total length usually less than 120 mm; length of skull usually less than 21 mm; postauricular patch conspicuous. Silky pocket mice:

Two species of silky pocket mice occur in Texas, but only specialists working with prepared study specimens can identify them.

(1) Perognathus flavus (silky pocket mouse) occurs in the Panhandle and Trans-Pecos portions of Texas.

(2) Perognathus merriami (Merriam’s pocket mouse) occurs in the Great Plains, central, and southern regions of Texas.

• Upper incisors plain, not grooved, on outer face; pelage spiny to touch: Liomys irroratus (Mexican spiny pocket mouse).
• Upper incisors distinctly grooved on outer face: 13

• Length of tail less than length of head and body (tail laid forward over back does not reach snout); weight 30 to 47 grams: Chaetodipus hispidus (hispid pocket mouse).
• Length of tail greater than length of head and body (tip of tail extends beyondsnout when laid forward): 14

• Rump with conspicuous black-tipped "spines"; tail sparsely haired on basal half; soles of hind feet blackish; upperparts grizzled blackish: Chaetodipus nelsoni (Nelson’s pocket mouse).
• Rump without conspicuous, black-tipped "spines.":

This category contains two species that only a specialist working with comparative material can identify with certainty.

(1) Chaetodipus penicillatus (desert pocket mouse) occurs in sandy soils mainly in Trans-Pecos Texas.

(2) Chaetodipus intermedius (rock pocket mouse) occurs mainly in rocky situations in the Trans-Pecos section of the state.

• Tail paddle-shaped, naked, scaly; hind feet webbed; size large: Castor canadensis (American beaver).
• Tail not paddle-shaped: 16

• Pelage with intermixed sharp quills; large, 4 to 12 kg: Erethizon dorsatum (porcupine).
• Pelage without quills: 17

• Lower jaw with four cheek teeth on each side: 18
• Lower jaw with only three cheek teeth on each side: 28

• Hind feet fully webbed; adults weigh up to 12 kg; tail long, naked, and nearly circular in cross section: Myocastor coypus (nutria).
• Hind feet not fully webbed: 19

• "Flying" membrane between front leg and hind leg on each side; color wood brown above, white below: Glaucomys volans (eastern flying squirrel).
• Legs normal, no "flying" membrane: 20

• Upperparts striped or distinctly spotted or both: 21
• Upperparts not striped or distinctly spotted: 25

• Upperparts striped: 22
• Upperparts spotted: 24

• One white stripe on each side; underside of tail grayish white (held over back while animal is running); upperparts grizzled grayish: Ammospermophilus interpres (Texas antelope squirrel).
• Three or more white or light stripes on upperparts: 23

• Six continuous, whitish stripes alternating with seven rows of whitish spots; ground color brown: Spermophilus tridecemlineatus (thirteen-lined ground squirrel).
• Four whitish stripes alternating with five dark brown stripes; sides of face striped: Tamias canipes (gray-footed chipmunk).

• Spots in 10 or more distinct rows; tail narrowly bushy and about three times as long as hind foot: Spermophilus mexicanus (Mexican ground squirrel).
• Spots scattered, never in distinct rows; tail about twice as long as hind foot: Spermophilus spilosoma (spotted ground squirrel).

• General color yellowish brown; tail very short (1.5 times length of hind foot) and black-tipped: Cynomys ludovicianus (black-tailed prairie dog).
• General color gray, brown, or blackish; tail long and bushy: 26

• Belly reddish or rusty in color; upperparts grayish; hind foot 70 mm or more: Sciurus niger (eastern fox squirrel).
• Belly whitish or grayish; not reddish; hind foot 70 mm or less: 27

• Belly white; upperparts gray, unspotted: Sciurus carolinensis (eastern gray squirrel).
• Belly grayish, back grayish with faint light spots, or shoulders and head black and rump grayish or brownish: Spermophilus variegatus (rock squirrel).

• Tail flattened laterally, sparsely haired and scaly; hind toes fringed with stiff hairs; length of adults about 45 cm: Ondatra zibethicus (common muskrat).
• Tail round, sparingly haired or bushy: 29

• Enamel pattern of molar teeth with transverse or oblique folds or triangles: 30
• Enamel pattern of molar teeth with two or three rows of cusps (unworn condition) or roughly circular with slight lateral indentations (worn condition): 37

• Mouse size, total length usually less than 150 mm; tail less than 50 mm; ears nearly hidden in the fur: 31
• Rat size, total length of adults 225 mm or more; tail 100 mm or more; ears conspicuous or partly hidden in the dense fur: 33

• Tail less than 25 mm in length; hind foot usually less than 18 mm; color glossy, reddish brown: Microtus pinetorum (woodland vole).
• Tail more than 25 mm in length; hind foot usually more than 18 mm; color brownish gray or blackish: 32

• Enamel pattern of third upper molar with no more than two closed triangles, often with no closed triangles, hence with three loops; never more than two inner re-entrant angles: Microtus ochrogaster (prairie vole).
• Enamel pattern of third upper molar with three closed triangles or, if with only two closed triangles, then with three inner re-entrant angles: Microtus mexicanus (Mexican vole).

• Ears conspicuous; tail in adults usually 150 mm or longer; eyes large, black, and bulging in life; fur rather soft; whiskers long, usually more than 50 mm (woodrats): 34
• Ears partly hidden in dense pelage; tail 100-125 mm long; pelage rather harsh; whiskers 25 to 35 mm long (cotton rats): 35

• First upper molar tooth with a deep antero-internal fold extending half-way across the crown: Neotoma mexicana (Mexican woodrat).
• First upper molar tooth without a deep antero-internal fold extending half-way across the crown:

This category includes three species that are difficult to identify without close examination of the skull and the baculum.

(1) Neotoma micropus (southern plains woodrat) occurs in the brushlands of the western and southern portions of the state. Over most of its range, this woodrat is characterized by a steel gray dorsum as compared to the brownish pelage of the other two species.

(2) Neotoma albigula (white-throated woodrat) is found in the western half of Texas.

(3) Neotoma floridana (eastern woodrat) is found in the eastern half of Texas.

• Underparts buffy to ochraceous; tail entirely black; top surface of feet buffy: Sigmodon fulviventer (tawny-bellied cotton rat).
• Underparts whitish and not buffy or ochraceous; tail bicolor, dark above and light below; top surface of feet whitish: 36

• Snout and eye rings yellowish or orangish and conspicuously different than color of backs and sides; hind foot of adults usually less than 30 mm; total length usually less than 260 mm: Sigmodon ochrognathus (yellow-nosed cotton rat).
• Snout and eye rings not conspicuous and same color as sides and back; hind foot usually more than 30 mm; total length usually more than 260 mm: Sigmodon hispidus (hispid cotton rat).

• Rat size, total length 230 mm or more: 38
• Mouse size, total length usually less than 200 mm: 40

• Cusps on upper molars in two rows; hind foot narrow and slender (rice rats):

There are two species of rice rats in Texas that only a specialist can identify with certainty.

(1) Oryzomys palustris (marsh rice rat), a grayish brown form characteristic of marshy areas along the coast from Brownsville northward into deep East Texas.

(2) Oryzomys couesi (Coues’ rice rat), a tawny form that occurs in marshy areas in extreme South Texas (Hidalgo and Cameron counties).

• Cusps on upper molars in three rows (introduced rats): 39

• Tail slender and as long as or longer than head and body (tail reaches to or beyond nose when laid forward); color brownish or black; weight to 225 g: Rattus rattus (roof rat).
• Tail chunkier and shorter than head and body; color brownish; weight to 450 g: Rattus norvegicus (Norway rat).

• Outer face of each upper incisor with deep groove (harvest mice): 41
• Outer face of upper incisors not grooved: 44

• Tail much longer than head and body (projects beyond nose when laid forward along back); last lower molar with dentine in the form of an "S:" Reithrodontomys fulvescens (fulvous harvest mouse).
• Tail shorter than or about as long as head and body; last lower molar with dentine in the form of a "C": 42

• Color rich brown to blackish brown; a distinct labial shelf or ridge, often with distinct cusplets on first and second lower molars: Reithrodontomys humulis (eastern harvest mouse).
• Color mainly grayish brown or light buff; no distinct labial shelf or ridge on first and second molars: 43

• Tail shorter than head and body. Breadth of braincase not exceeding 9.6 mm: Reithrodontomys montanus (plains harvest mouse).
• Tail length about equal to, or slightly longer than, head and body. Breadth of braincase of adults usually over 9.5 mm: Reithrodontomys megalotis (western harvest mouse).

• Upper incisors with distinct notch at tip when viewed from the side; distinctly musky odor: Mus musculus (house mouse).
• Upper incisors lacking distinct notch at tip: 45

• Total length of adults 100 mm or less; tail short, 35 mm, about three times length of hind foot; color blackish or sooty: Baiomys taylori (northern pygmy mouse).
• Total length of adults 125 mm or more; color not blackish or sooty: 46

• Tail less than 60% of head and body; coronoid process of mandible extends high above level of condyloid process; soles of feet furred (grasshopper mice): 47
• Tail more than 60% of head and body; coronoid process of mandible does not ascend above tip of condyloid process; soles of feet only slightly furred (deer mice and relatives): 48

• Tail less than half length of head and body; crown length of maxillary toothrow 4.0 mm or more: Onychomys leucogaster (northern grasshopper mouse).
• Tail more than half length of head and body; crown length of maxillary toothrow 3.9 mm or less: Onychomys arenicola (Mearns’ grasshopper mouse).

• General color golden yellow: Ochrotomys nuttalli (golden mouse).
• General color brown, buff, or gray (white-footed mice): 49

• Tail much shorter than head and body: 50
• Tail as long as or longer than head and body: 52

• Hind foot (of adults) greater than 23 mm: Peromyscus gossypinus (cotton mouse).
• Hind foot (of adults) less than 23 mm: 51

• Tail with narrow and distinct dorsal stripe; total length of adults usually less than 170 mm; length of tail usually less than 75 mm; greatest length of skull usually less than 26 mm: Peromyscus maniculatus (deer mouse).
• Tail with broad dorsal stripe and not sharply bicolored; total length of adults usually more than 170 mm; length of tail usually more than 75 mm; greatest length of skull usually more than 26 mm: Peromyscus leucopus (white-footed mouse).

• Nasals decidedly exceeded by premaxillae; two principal outer angles of first and second upper molars simple, without (or at most with rudimentary) accessory cusps or enamel lophs; sole of hind foot naked to end of ankle; no pectoral mammae; inguinal mammae, 2-2: Peromyscus eremicus (cactus mouse).
• Nasals slightly or not at all exceeded by premaxillae; two principal outer angles of first and second upper molars with well-developed accessory tubercles or enamel lophs; sole of hind foot hairy on proximal fourth to ankle; pectoral mammae, 1-1, inguinal mammae, 2-2: 53

• Ear longer than hind foot; tail about as long as head and body (except in P. t. comanche in which it is longer); bullae unusually inflated: Peromyscus truei (piņon mouse).
• Ear equal to or shorter than hind foot; tail usually longer than head and body; bullae moderately or less inflated: 54

• Hind foot length of adults more than 24 mm: Peromyscus attwateri (Texas mouse).
• Hind foot length of adults less than 24 mm: 55

• Tarsal joints of ankles white like upper side of hind foot; baculum with long cartilaginous spine at its terminal end: Peromyscus pectoralis (white-ankled mouse).
• Dusky color of hind leg extending to end more or less over tarsal joint, baculum with a short cartilaginous spine at its terminal end: 56

• Dorsal coloration grayish black, and often like immature pelage; top of head and flanks of adults predominantly grayish; first two lower molars usually with one or more accessory lophids or stylids: Peromyscus nasutus (northern rock mouse).
• Dorsal coloration with considerable yellow or buff; top of head same color as back; flanks of adults predominantly bright yellowish brown; first of two lower molars usually without any accessory lophids or stylids: Peromyscus boylii (brush mouse).