Streptacis Meek 1871

Streptacis ? khaokhadensis sp. nov.

(Fig. 25 O–R)

Etymology. After the Khao Khad Formation in which the studied gastropod material was found.

Holotype. ESKU-19-LP 213.

Paratype. ESKU-19- LP 214.

Dimensions (mm): ESKU-19- LP 213: height = c. 2.4; width = c. 0.8. ESKU-19- LP 214: height = c. 1.3; width = c. 0.7.

Type locality and stratigraphical range. Erawan Hill, Chong Sarika sub-district, Phatthana Nikhom district, located about 13 km east of Lopburi Province, Central Thailand (Fig. 1), Khao Khad Formation, Saraburi Group, Middle Permian, Roadian.

Description. Shell very small, high-spired, slender, largest specimen consisting of 7 whorls; protoconch smooth, seemingly coaxially heterostrophic; whorls low, smooth; earliest teleoconch whorls low, markedly convex with periphery at mid-whorl; later whorls higher, less convex; sutures deep; base and aperture unknown.

Remarks. Both specimens at hand are close to the Permian Japanese species Knightella hydrobiformis Nützel, 2012 from the Akasaka Limestone as reported by Nützel & Nakazawa (2012) in shell shape and size. However, in K. hydrobiformis convexity of the whorl face and height of the whorl does not change during ontogeny in contrast to the specimens at hand. The protoconch of the present material shows coaxial heterostrophy as does a specimen representing K. aff. hydrobiformis illustrated by Nützel & Nakazawa (2012, fig. 20N, O). However, the holotype of K. hydrobiformis does not display heterostrophy (maybe due to preservation) and was hence assigned to the caenogastropod genus Knightella (see Nützel 1998). Streptacis is characterized by transaxial heterostrophy i.e., the protoconch axis is more or less perpendicular to the shell axis of the teleoconch. However, Streptacis ? khaokhadensis sp. nov. shows coaxial heterostrophy as does for instance the Jurassic genus Usedomella Gründel, 1998 and the Pennsylvanian Mapesella Bandel, 2002 a. However, in Usedomella the width of the protoconch exceeds that of the early teleoconch and Mapesella has axial threads on the teleoconch whorls, both is not the case in S.? khaokhadensis sp. nov.. Therfore we tentaively place S.? khaokhadensis sp. nov. in Streptacis. S.? khaokhadensis sp. nov. differs from K. irregularis (Longstaff, 1933) in whorl profile that is somewhat pendent and in being larger.