Description of holotype (Fig. 4A). Scalation and proportions as in the diagnosis for the genus. Individual characteristics include: internasals much wider than long angled posteriorly; prefrontals about 1 times wide as long, medial suture slightly offline so that right prefrontal contacts left internasal; single preocular, much wider than high, in contact with nasal; two postoculars, the upper larger and lower much smaller; frontal wider anteriorly, gradually narrowing to rounded point between parietals; parietals almost equal size, right parietal slightly wider; temporals above fifth supralabials contacting both postoculars; five supralabials, first and second much the smallest, third higher in contact with orbit, fourth under orbit in contact with lower postocular and fifth very large; mental triangular between first infralabials; seven infralabials; anterior chin shields in broad contact with infralabials one to three; posterior chin shields in narrow contact with third infralabial only; six rows of intergulars between chin shields and anteriormost broad ventral.

After ≥ 167 years in preservative (Fig. 4A), the holotype has faded to white on the dorsal surface, including tip of snout and interspace between dark bands, with indistinct brown apexes extending forward as fine edge on posterior facets of scales along the body, gradually fading posteriorly to tail tip. Head and nape bands have faded to brown.

Colour in life and in preservative. The following description of colour in life is based on Figs. 5A, B, C, D and field observations of N. bimaculatus in Western and South Australia. The descriptions in preservative and of variation are based on all specimens of N. bimaculatus listed in the Appendix.

In life, variable body colour of shades of reddish brown (Figs. 5A, B), yellowish brown (Fig. 5C), orange brown or bright red (Fig. 5D), with paler interspaces on the head or similar to body colour (Figs. 5A, B, C, D), with pale cream to yellowish base or spots along the body, and dark pigment consisting of dark brown or black narrow to wide margins (almost covering ½ of scale) on the posterior facets, grading to reduced margins or smudging on tail. Dark margins varying in their intensity from clearly defined forming a reticulated pattern (Figs. 5A, B), to relatively indistinct with margins mostly restricted to apex, particularly in pale individuals (Fig. 5C). Occasionally without any indication of dark pigment on the body accentuating the pale bases or spots which tend to be larger lateroventrally (Fig. 5D, see also Storr et al. 2002: 130, Plate 30; Allen & Vogel 2019: 92, Plate RS15693‒4). Black tip on the snout present (Figs. 5A, B, D) or absent (Fig. 5C). When present extending back before posterior point of rostral often extending on to ventral edge, and lateral edge partially or narrowly covering internasals ending on nasals. Black band on head commencing on or just before anterior edge of frontal and just forward of eyes, extending back partially or narrowly covering supralabials two to five and ending on or just before posterior edge of parietals. Black band on nape transverse and 3½‒6 scales long, separated from head band by 2‒4½ pale vertebral scales. Occasionally with an indication of a third black blotch on the forebody (Fig. 5A) or reduced black band on the head (Fig. 5C). Eyes are black without discernible pupils. Ventral surface under the head and along the body, including the lateroventral edges, is cream white with glossy shine.

In preservative, body colour fades to white including interspaces on the head. Dark head and nape bands, tips on snouts (when present) and posterior margins on body scales fade to brown. The dark posterior margins on body scales can either be distinct (e.g. SAMA R59348, SAMA R61148, WAM R112153, WAM R118840, WAM R169196) or largely indistinct in the majority of specimens (e.g. SAMA R61174, SAMA R69322, WAM R60628, WAM R123596, WAM R135079), tending to be clear lateroventrally and grading to smudging on tail. The white ventral surface remains glossy.

Storr (1968: 86) noted geographic variation in colour and patterning between coastal and inland groups representing 45 specimens but his observations are incongruent with the intra-specific genetic variation observed (Fig. 2). Storr observed inland specimens differed in the dark tip on the snout being always present, larger head band, smaller nape band and darker body colour. We observed little correlation between these characters and the coastal and inland groups. Using 134 WAM specimens of N. bimaculatus (including those listed in the Appendix), we found the dark tip was present in 42% of individuals from the western and 87% from the eastern parts of range, while the tip was light in 58% of specimens from western parts of the range and only 13% in eastern parts. Five specimens had only a weak indication of a dark spot on the snout. In the South Australian material (see Appendix), a dark tip on the snout is present on all individuals except one specimen (SAMA R61174), the dark head band mostly ended on or just before posterior edge of parietals (e.g. SAMA R61147‒48) and the posterior margins on body scales varied from indistinct smudges (e.g. SAMA R61174, SAMA R69322) to distinct wide margins (e.g. SAMA R59348, SAMA R67492).

The dark head band ending behind posterior edge of parietals was present in 27% of specimens from eastern parts of range including occasional individuals with a third dark blotch on the forebody (e.g. WAM R74220, WAM R147073, WAM R165881, WAM R169106). By contrast, in the western parts of range, we recorded the dark head band typically ending on or just before posterior edge of parietals in 61% of specimens and individuals with a third blotch were rarely seen (WAM R60628, WAM R62367). We observed no appreciable variation in the nature of the nape band between coastal and inland specimens.

Our field observations suggest the degree of the darker body colour in eastern parts of range is affected by the intensity of the posterior margins on scales, and arguably influenced by the subtly different substrates inhabited by these highly fossorial snakes. Individuals from the coastal sands and plains are paler in general appearance when compared to slightly darker individuals occurring on inland sands, a phenomenon regularly observed in other fossorial reptiles (Maryan et al. 2013).

Measurements, counts, and scalation. Table 2 presents the minimum‒maximum range, means and standard deviations of the characters measured and counted (as defined in Table 1) for each sex and all adult specimens of N. bimaculatus listed in the Appendix. Sexual dimorphism is pronounced, where males have longer tails (as found previously by Clarke & How 1995) and higher subcaudal scale counts (Table 2). The widest part of the head is broader in females, indicated by broader total width, broader posterior frontal scale and longer parietal scales (Table 2, Fig. S3).

The head scale configuration in N. bimaculatus display minimal intraspecific variation. For illustrations of head scalation in N. bimaculatus, see Greer (1997: 169) and Storr et al. (2002: 190). All the specimens examined have the nasal contacting the preocular, except for two specimens (WAM R82736, WAM R125965) in which these scales are separated by the prefrontal on both sides and one specimen (SAMA R61147) on the right side. Occasional specimens have the second supralabial contacting the preocular on both sides (WAM R719, WAM R54543, WAM R104313, WAM R172498, WAM R173341). Storr (1968: 85) recorded the typical condition of the upper primary and secondary temporals fused to form a single elongate scale in 92% of specimens. We observed the same typical condition in all specimens on both sides, except in seven specimens: WAM R82736 has 1 primary + 1 secondary temporals on both sides (the secondary is much smaller), SAMA R61148 and WAM R62367 on the left side and WAM R4722, WAM R125965, WAM R135079, WAM R147076 on the right side.

Other variations of scalation include: one specimen (SAMA R61174) has divided prefrontals; one specimen (SAMA R61147) has four supralabials on the left side, caused by fusion between first and second supralabials and the third supralabial is fused with preocular; two specimens (WAM R147073, WAM R147076) have six supralabials on the right side, caused by an extra scale interposed between fourth and fifth supralabials; three specimens (WAM R65292, WAM R123596 on the right side, WAM R172498 on the left side) have one postocular, caused by either fusion of lower with fourth supralabial or upper with parietal; one specimen (WAM R165295) has the temporal fused with fifth supralabial on the right side; two specimens (WAM R125965, WAM R165295) have the temporal reduced in size on the left side, caused by the fifth supralabial contacting parietal; one specimen (WAM R135646) has a combination of both divided and undivided subcaudals and a specimen (WAM R60628) has exceptionally high ventral and vertebral scale counts of 263 and 273, respectively.