Pareidae-Check list snakes northern Thailand

Pareidae (Snail Eaters or Slug Snakes)
Family: Pareidae. Rather modestly-sized (30-70 cm) snakes with blunt heads, mainly active at night. A central groove is lacking between chin shields. Four species are known to occur in northern Thailand.

Pareas carinatus or Keeled Slug Snake. Upperside yellowish-brown or orange-brown, often with a fine dark reticulation. Body laterally compressed. Red eye with black vertical eliptic pupil. Common throughout much of northern Thailand in forested or semi-cultivated hills at elevation of 300-900 m asl. A-B: Specimens from Tha Song Yang District, Tak, in evergreen forest at about 700-800 m asl. C: Details of the head scalation of a specimen from Mae Chaem District, Chiang Mai, showing a relatively large supraorbital and preorbital and a prefrontal shield (asterix) that does not enter the orbit. D: A slightly paler specimen from Mae Chan District, Chiang Rai. E: A brightly colored juvenile from Chiang Dao District, Chiang Mai.

Pareas hamptoni or Hampton’s Slug Snake. This snake superficially closely resembles Pareas carinatus but can be distinguished from the latter by its somewhat larger size, its larger number of ventral shields and the prefrontal shields that enter the orbit. It occurs in evergreen forest at high elevation (1000-1700 m asl). A. A specimen from Khun Yuam District, Mae Hong Son. B-C. A specimen from Chiang Mai’s Mae Wang District.

Pareas macularius, the Keeled Bicolored-spotted Snail Eater.
This species has often been confused with Pareas margaritophorus, as its ground color is also grey and it is also ornamented with black-and-white spots. However, there are many differences: Pareas macularius is larger (specimens measuring more than 60 cm have been recorded), has 5-9 rows of dorsals that are keeled, has a differently shaped nuchal color of which the color is never yellow, pink or orange. The 7th, largest spoon-shaped supralabial nearly always has a large intensely black blotch (lacking in Pareas margaritophorus). This snake is common throughout the mountains of northern Thailand in forested or cultivated areas at elevations of 800-1700 m a.s.l. A. A juvenile from Khun Yuam district, Mae Hong Son. B-C: Two different specimens from Mueang Pan District, Lampang, showing the considerable variation in the shape of the finely speckled nuchal color. D.Details of the head and nuchal collar of a specimen from Phu Hin Rong Kla National Park, Nakhon Thai District, Phitsanulok.
This species had been erroneously synonymized with Pareas margaritophorus (below) in 2004, but it was resurrected as a valid species recently (April 2017). You can download a PDF of the article on this revalidation here:

Pareas margaritophorus, the Smooth Bicolored-spotted Snail Eater.
This grey little snake (max. length 45 cm) is ornamented with many bicolered (black-and-white) spots the size of one dorsal scale. All dorsal scales are smooth. Most specimens have a nuchal collar that can be entire or tripartite which color varies from white and pale sulphur to pink or bright orange. This species is common throughout northern Thailand where it prefers forested or cultivated areas at elevations of 300-700 m. A-B: Specimens from Tha Song Yang District, Tak. C: Specimen from Bo Kluea District, Nan, D: Specimen from Mae On District, Chiang Mai.