Holotype.VNUF R.2021.01 (Field no. LC17.01), adult male, collected by H.M. Ly on 25 August 2017 from Lung Cu Commune, Dong Van District, Ha Giang Province (23°21'N, 105°19'E, at an elevation of 1391 m a.s.l.). Paratypes.IEBR R.5151 (Field no. LC17.02), VNUF R.2021.03 (Field no. LC17.03), adult males, IEBR R.5152 (Field no. ST17.01), VNUF R.2021.08 (Field no. HG17.08), adult females, collected at the same locality as holotype on 25 August 2017; two adult females, VNUF R.2023.01 (Field no. LC22.01) and VNUF R.2023.02 (Field no. LC22.02), collected at the same locality as the holotype on 5 August 2022.

Hemiphyllodactyluslungcuensis sp. nov. can be distinguished from its congeners by a combination of the following characters: a maximum SVL of 44.2 mm; trunk not particularly elongate (AG/SVL ratio 0.52); chin scales 8–10, distinct enlarged postmentals; circumnasal scales two or three; supralabial scales 11 or 12; infralabial scales 10 or 11; ventral scale rows 6–9; dorsal scale rows 12–16; digital lamellae formula 4(3)–4–5(4)–4 (forefoot) and 4–5–5–5 (hindfoot); precloacal pores 17–25 and continuous series in males, 0–22 pitted precloacal scales in females; cloacal spur single in both sexes; enlarged subcaudal scales absent; dorsal surface brown sand color, with irregular dark brown streaks; a distinct dark postorbital stripe extending to at least the base of the neck, uneven dark streaks running along the flanks and ending at the base of tail, a cream-colored V-shaped postsacral mark with anteriorly projecting arms; a pale yellow ventral view of the body and limbs; and unpigmented caecum and gonads.

Adult male, SVL 39.2 mm; tail length (TaL 37.3 mm); body compressed (TrunkL 20.3 mm); head triangular in dorsal profile, depressed, distinct from neck, and head longer than wide (HeadL 11.0 mm, HeadW 8.0 mm); eye large, pupils vertical (EyeD 1.8 mm); snout-eye distance (SnEye 4.3 mm); snout moderate, round in dorsal profile, longer than eye diameter (EyeD/SnEye 0.42); nare-eye distance (NarEye 3.2 mm); internarial distance (SnW 1.5 mm); ear opening oblique (EarD 0.7 mm).

Proportions: EarD/EyeD 0.39, TrunkL/SVL 0.52, HeadL/SVL 0.28, HeadW/SVL 0.20, HeadW/HeadL 0.73, SnEye/HeadL 0.39, NarEye/HeadL 0.29, EyeD/HeadL 0.16, SnW/HeadL 0.14, EyeD/NarEye 0.56, SnW/HeadW 0.19.

Scalation: Rostral large, wider than high, partially divided dorsally, bordered posteriorly by two large supranasals and two internasals (SnS); nares in contact with rostral anteriorly, first supralabial ventrally, supranasal dorsally, and three nasals posteriorly on each side; supralabial scales 12/12 (left/right), infralabial scales 11/10 (left/right), gradually decreasing in size towards angle of jaw; scales on head small, round, largest on rostrum; mental triangular, bordered by first infralabials and posteriorly by two enlarged postmentals, eight chin scales touching internal edge of infralabials and mental between the juncture of the second and third infralabials on each side of the head, anterior pair enlarged; ventral scales smooth and flat, much larger than dorsal scales, subimbricate, in seven rows (contained within one eye diameter); dorsal scales small, granular in 13 rows at midbody on dorsum (contained within one eye diameter), enlarged tubercles absent; fore-limbs relatively short, covered dorsally with granular, subimbricate scales, smaller smooth scales ventrally; 25 pore bearing femoral and precloacal sacles, in a continuous row; terminal two phalanges free, claws absent on first finger and on first toe,large triangular lamellae present on the second to fifth digits of both the forefoot and hindfoot pads, with digital formula 4–4–5–4 (forefoot) and 4–5–5–5 (hindfoot); four on first fingers, four on first toes; cloacal spurs (CloacS) one on each side; semi-regenerated tail; caudal scales flat, not forming distinct caudal segments; caecum and gonads unpigmented.

Dorsal surface of head, body, and limbs brown sand color, with irregularly shaped dark-brown streaks; a dark postorbital stripe, distinct, extending to at least base of neck; uneven dark streaks running along the flanks and ending at the base of tail; two dark dots on dorsal surface of tail; dorsal surface of tail has dark bands evenly spaced from base of tail to tip of tail; postsacral mark cream-colored and V-shaped, bearing anteriorly projecting arms; ventral surface of body and limbs pale yellow; subcaudal region pale peach.

The scale counts vary among the type series: chin scales 9 or 10; scales between supranasals one or three; supralabials 11; infralabials 10; ventral scale rows seven or nine; 0–22 pitted precloacal scales in females (distinct pore-bearing scales in males). Dorsal surface of the tail has dark bands evenly spaced from the base of the tail to the tip of the tail in females. Subcaudal region of original part of tail of the adult male paratype (IEBR R.5151) is reddish-orange, while the regenerated portion of the tail is uniformly dark grey. For other morphological characters of the paratypes are given in Table 8.

Hemiphyllodactyluslungcuensis sp. nov. is currently known only from the type locality in Lung Cu Commune, Dong Van District, Ha Giang Province, northeastern Vietnam (Fig. 1).

Specific epithet lungcuensis is a toponym in reference to the type locality of the species. For the common names, we suggest Lungcu Slender Gecko (English) and Thạch sùng dẹp lũng cú (Vietnamese).

The species is known from a karstic habitat. The type series were found on karst cliffs between 19:00 and 20:30, approximately 1–1.5 m above the ground. The surrounding habitat was a disturbed karst forest with small hardwood and shrub trees, and was located near residential areas and road systems (Fig. 9). The type locality of the new species is located in an unprotected area, and thus we need to clarify the impact and threats of the localized disturbance so as to put in place effective conservation measures.

The molecular analyses indicate that Hemiphyllodactyluslungcuensis sp. nov. is a phylogenetically distinct lineage of the clade 6 that is not embedded within any lineage of the other 13 species of clade 6 (Agung et al. 2022) and bears an uncorrected pairwise sequence divergence of 4.6–20.2% from them (Fig. 1, Table 2). Hemiphyllodactyluslungcuensis sp. nov. is the sister species to a clade composed of H.huishuiensis and H.yanshanensis (Fig. 1) from which it bears an uncorrected pairwise sequence divergence of 5.3% from H.huishuiensis and 6.9% from H.yanshanensis (Table 2). Hemiphyllodactyluslungcuensis sp. nov. differs significantly from H.huishuiensis in mean values of EyeD (0.31 vs. 0.41, p = 0.001) and SnW (0.23 vs. 0.08, p = 0.001) and from H.yanshanensis in mean values of TrunkL (1.31 vs. 1.29, p = 0.03) and EyeD (0.31 vs. 0.38, p = 0.002). In addition, Hemiphyllodactyluslungcuensis sp. nov. differs significantly from H.nahangensis in mean values of EyeD (0.31 vs. 0.42, p = 0.001), SnW (0.23 vs. 0.05, p = 0.001), DS (14.75 vs. 21, p = 0.001), and SL1T (4 vs. 3, p = 0.001) (Tables 6, 7). Statistically significant differences of morphometric and meristic traits between Hemiphyllodactyluslungcuensis sp. nov. and all other species in clade 6 are presented in Tables 6, 7. Due to the unavailability of individual specimens of H.dushanensis for statistical analysis, we obtained integrated data of this species from Do et al. (2020: table 4) for comparison, the new species can be distinguished from H.dushanensis by the presence of dark postorbital stripe, dark dorsolateral stripe, and dark dorsal transverse blotches (vs. absent in H.dushanensis).