Gonaxis denticulatus (Dohrn, 1878)

Muratov, Igor V., 2010, Terrestrial molluscs of Cabo Delgado and adjacent inland areas of north-eastern Mozambique, African Invertebrates 51 (2), pp. 255-255 : 272

Gonaxis denticulatus (Dohrn, 1878)

Fig. 33

Streptaxis denticulatus: Dohrn 1878: 152 ( Mombasa, south-eastern Kenya; c. 7.0× 4.5 mm).

A widespread, common East African species with a general distribution from Kenya to Mozambique. Found at eight stations inland. This species is smaller and has a less distorted axis than the preceding species. It differs from other similarly sized Gonaxis by the smooth shell with a strongly serrated suture.

Dohrn had a quite unusual way of measuring distorted shells. He gave “Diam. maj. 7, min. 4½, alt. 3½ mill.” in the original description of denticulatus . Apparently, he let the shell lie down freely on the flat surface and measured the height of the shell above the surface, calling that measurement “alt.”, which corresponds to the minor diameter in modern terminology. Then his “diam. maj.” roughly corresponds to the height (being slightly larger) and “diam. min.” roughly corresponds to the major diameter (being slightly smaller). Thus, the size from the original description in modern terminology would be: height ~ 7 mm, diameter major ~ 4.5 mm and diameter minor – 3.5 mm. (This became apparent in the original description of the similarly sized but much stronger sculptured Streptaxis kirkii Dohrn, 1865 , where he added the diameter of the aperture: “Diam. maj. 5/16, min. 3/16, alt. 3/16, apert. diam. 1/8 poll.” ( Dohrn 1865: 232), which corresponds to: height – 7.94 mm, diameter major and minor – 4.76 mm, aperture diameter – 3.18 mm in modern terminology.)

Surprisingly, von Martens (1897: 30, 32) copied the dimensions given by Dohrn into his comparison table, which is probably the reason why he listed separately the very similar Streptaxis ordinarius Smith, 1890 . The latter species measures 6.5×4.0 mm in the original description (Smith 1890: 160, pl. 6, figs 2 and 2a) and was synonymised with Streptaxis denticulatus by Thiele (1911: 186), who apparently compared the types of both species.

The similarly sized Gonaxis gibbonsi Taylor, 1877 (7.0× 4.4 mm) has a proportionally more elongated and slightly more distorted shell. G. cressyi Connolly, 1922 is smaller (6.2× 3.8 mm) and has slightly less distorted shell. G. mozambicensis ( Smith, 1881) is slightly larger: 7.5× 4.5–4.8 mm ( Smith 1881: 280, pl. 32, figs 6, 6a) but may prove to be a synonym of denticulatus .

Figures

Figs 30–38.Streptaxidae:(30) Edentulina affinis,33.4×17.6 mm,stn 5; (31) Taylorialeroyi, 14.4×21.0 mm, stn 5; (32) Gonaxis cf. percivali, 9.4×6.9 mm, stn 1; (33) G. denticulatus, 7.0×4.6 mm, stn 11; (34) Gulella sexdentata var. liederi, 9.4×4.8 mm, stn 1; (35) G. subhyalina, 6.5×2.3 mm, stn 1; (36) G. perissodonta, 3.2×1.5 mm, stn 7; (37) Gulella delgada sp. n., 4.0×1.8 mm, 1st syntype, stn 7; (38) Streptostele herma, 3.8×1.2 mm, stn 7. Scale bar 10 mm in Figs 30–31; 3 mm in Figs 32–34; 1 mm in Figs 35–38.