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Trimorphodon lambda COPE, 1886

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Higher TaxaColubridae, Colubrinae, Colubroidea, Caenophidia, Alethinophidia, Serpentes, Squamata (snakes)
Common NamesE: Sonoran Lyre Snake
G: Sonora-Lyraschlange, Westliche Lyranatter
S: Ilamacoa de Sonora 
SynonymTrimorphodon lambda COPE 1886: 286
Lycodon lyrophanes COPE 1860
Trimorphodon lyrophanes — COPE 1875: 38 (part.)
Trimorphodon vandenburghi — COWLES & BOGERT 1936: 41
Trimorphodon lambda lambda — DIXON et al. 1962: 98
Trimorphodon lambda — GEHLBACH 1958
Trimorphodon biscutatus lambda — GEHLBACH 1971: 208
Trimorphodon biscutatus lambda — SCOTT & MCDIARMID 1984
Trimorphodon biscutatus lambda — TANNER 1985: 660
Trimorphodon biscutatus lambda — CROTHER 2000
Trimorphodon lambda — DEVITT et al. 2008
Trimorphodon lambda — WALLACH et al. 2014: 744 
DistributionMexico (Sonora, Chihuahua)
USA (Nevada, Arizona, S Utah, New Mexico)

Type locality: “Guaymas, Sonora”

vandenburghi: Type locality: “Wildwood Ranch near Ramona, San Diego County, California (elev. 1520 feet)”.  
TypesHolotype: USNM 13487, juvenile 
DiagnosisDEFINITION AND DIAGNOSIS (genus). A colubrid snake genus with lateral head scales fragmented, numerous, and variable in number, loreals generally 2 or 3 (2-5), preoculars generally 3 or 4 (2-5), postoculars generally 3 (2-4), temporal generally 2 or 3 (1-5) + 3 or 4 (2-5); supralabials generally 8 or 9 (7-10), infralabials generally 11 or 12 (9-14); dorsal scales smooth (or bluntly keeled in some males) with paired apical pits, generally in 21-25 rows at midbody with posterior reduction; anal plate divided or single; subcaudals paired; head distinct from body; eye moderate to large with vertically elliptical pupil; body and tail moderately slender to very slender and laterally compressed; length to over 1500 mm; color pattern composed of black or brown blotches, usually with pale centers which tend to divide the primary blotches into secondary blotches which in turn may be divided; small blotches sometimes present dorsally between large blotches or laterally in a row; head pattern usually complex with broad pale collar (tau) or pale chevron or lyre (most biscutatus) on the nape; pale interocular bar often present; ground color brown, tan, gray, or brick red with juvenile color pattern more intense and sharply defined; venter paler than dorsum and noticeably opalescent, may be spotted or mottled. Maxillary teeth 10-12, anterior 2 or 3 much larger than others which gradually decrease in size posteriorly to diastema which is followed by 1 or 2 enlarged teeth, deeply grooved on anterior face; anterior mandibular teeth enlarged; Duvernoy's gland well developed. (Scott & McDiarmid 1984)

Vertebrae (genus) short, broad and flat; wider than long, neural spine low and thin; haemal spine barely indicated as a slightly raised keel. Hypapophyses blade-like, present on anterior vertebrae only. (Scott & McDiarmid 1984)

Hemipenis (genus) 14-25 subcaudals long, single, and attenuate; sulcus single, reaching to tip; basal quarter naked, with or without tiny spinules; next distal fifth with thickened sulcal lips and covered with large spines and spinules, or naked (specimens from southern and Baja California); this part followed by two or three pockets on asulcate side with naked pouches and thick, spinulate lips; these pockets followed by a short naked neck that ends in another pocket under the distal portion of the organ, which is finger-shaped and covered with rows of spinulate papillae. (Scott & McDiarmid 1984)

Comparisons (genera): In North and Central American colubrid snakes, an undivided sulcus spermaticus, elliptical pupil, generally smooth scales, and enlarged grooved posterior maxillary teeth preceded by a diastema, define three genera: Leptodeira, Imantodes, and Trimorphodon (Dunn, 1928). Trimorphodon is further distinguished by having more than one loreal, a pocketed, non-capitate hemipenis, and slightly oblique scale rows (Duellman, 1958). (Scott & McDiarmid 1984)

DEFINITION (lambda). Primary dark body blotches 20-35, 4-8 scales wide, central pale spots may entirely divide body blotches, the resulting secondary blotches undivided; pale chevron on back of head with apex on posterior tip of frontal, not connected to interorbital bar; light spot on nape present or absent; snout may be mottled or pale without transverse light bar; ventrals less than 244; anal divided. (Scott & McDiarmid 1984) 
CommentType species: Lycodon lyrophanes COPE 1860 is the type species of the genus Trimorphodon COPE 1861.

Habitat: partly arboreal (Harrington et al. 2018). 
EtymologyNamed after the chevron-shaped mark on the head which looks like a Greek lambda.

Trimorphodon refers to the three tooth shapes in the upper jaw: the long, recurved anterior teeth, the shorter middle teeth, and the elongate, grooved fangs at the rear. The gender is masculine. 
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