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A single shell from the Misuku Hills with seven-fold apertural dentition: Gulella crux spec.nov.
A single smooth and small shell with a seven-fold apertural dentition from the Misuku Hills in northern Malaŵi, a source of many novelties among the land snails, has long defied identification. It obviously does represent a new species, which is now described below.
Gulella crux spec. nov.
(figs 12-13)
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Figs 12-13. Holotype shell of Gulella crux n. sp., front and side views (height 2.7 mm, RMNH Mol. 131917), Malaŵi, Mughesse Forest. Scale bar 1 mm. Fig. 12a shows details of aperture (highly enlarged), shell slightly tilted. Photography by Dr A.J. de Winter, layout by Ms Sancia E.T. van der Meij. Material.— Malaŵi, Chitipa Dist., Misuku Hills, Mughesse evergreen forest, leaf litter, c. 1800 m, 12.ix.1986, leg. Mrs Isobyl la Croix (RMNH Mol. 131917, holotype, figs 12-13). |
Diagnosis.— A species of Gulella s.l. characterized by a small (< 3.0 mm), smoothish (i.e. with faint costulation), shell with seven-fold apertural dentition consisting of parietal denticle, angular lamella, labral complex, two basal denticles, and a double columellar lamella.
Description.— Shell (figs 12-13) comparatively small (< 3 mm), subcylindrical, greatest width about the middle, under twice as long as wide, transparent when fresh. Umbilicus open but narrow. Spire produced, sides subparallel, not convex, apex flattened, obtusely conical to mamillate, smooth and very slightly pitted. Whorls six, slightly convex, seemingly smooth but with vague and fine costulation, most noticeable behind the labrum. Sutures impressed, filiform and hardly subcrenellate. Aperture subovate to squarish, about as high as wide, peristome slightly incrassate and reflected, somewhat obstructed by seven-fold dentition: a strong, almost perpendicular, inrunning angular lamella on the right of the paries; a large, almost bulbous labral complex corresponding to an outside depression behind the labrum, forming a large sinus with the angular lamella; an insignificant deeply situated process in the right basal region, more or less hidden by the labral complex; a centre-left basal process; a superficial mid-columellar denticle; a more deeply situated mamillate columellar complex; a fairly small, hardly superficial, mid-parietal denticle.
Measurements of shell: 2.7 × 1.4 mm, l/d 1.90, length last whorl 1.5 mm, aperture height × diameter 0.9 × 0.9 mm, whorls 6.
Animal.— Unknown.
Distribution.— So far this taxon has only been recorded from the Mughesse Forest in the Misuku Hills in northern Malaŵi.
Ecology.— G. crux n. sp. has only been reported from leaf litter in montane forest at c. 1800 m.
Derivatio nominis.— crux, a Latin noun (i.e. a noun in apposition), meaning ‘cross’, referring to the name of the collector Mrs Isobyl la Croix (‘croix’ being French for ‘cross’), one of Ms. Meredith’s star collectors in northern Malaŵi.
Discussion.— Only a single shell is available for study. In Verdcourt’s system (see above) the formula for the apertural dentition would be 2; 1; 2; 2. At first sight identification with G. perissodonta (Sturany, 1898) might be a possibility.
Some information is available on G. perissodonta (Van Bruggen, 1969: 46-50; 2006: 123-125; Herbert & Kilburn, 2004: 184-185), which, indeed, is a variable taxon. The shells are subject to variation in size (length 2.7-6.2 mm, i.e. the largest shells are more than twice as long as the smallest), shape (l/d 1.74-2.47, i.e. shell shape varies from squat and obese to slender and cylindrical), and pattern of apertural dentition (maximum 9-fold, but may be reduced to 8-fold or even 7-fold). The single shell of the new species G. crux would easily fit in here because its length (2.7 mm), shape (l/d 1.90), and apertural dentition (7-fold) are fully covered by the variability of G. perissodonta. The individual processes in the aperture of G. crux, however, are slightly different from those of G. perissodonta; there is no sinular denticle and the sinus is seemingly proportionally larger, the labral complex consists of a single large process, the deeply situated (single) basal denticle is insignificant, and the superficial mid-columellar denticle is quite well-developed and prominent.
What remains is the sculpture of the whorls and its attendant sutures. For G. perissodonta this has been described by Van Bruggen (1969: 46) as “Whorls . . . sculptured with close, oblique costulae; sutures from shallow to fairly deep, crenellate.” In G. crux the whorls are seemingly smooth but with vague and fine constulation, and the sutures are impressed, filiform and hardly subcrenellate. Also, the single shell of the latter has already 6 whorls while a shell of that size in the former would always have at the most 5½ whorls.
Close comparison with a series of G. perissodonta in the Leiden Museum shows that on the whole the differences between this and the new species are consistent. Apart from the size difference (i.e. very few shells of the former are as small as that of G. crux) and the sculpture of the whorls, the outer columellar process is always much less significant in G. perissodonta. The overall general impression is that the shell of G. perissodonta is noticeably ‘coarser’ than that of G. crux.
Biogeographically, in view of the known distribution of G. perissodonta (northernmost localities: Cabo Delgado and Vumba Is. on the coast of Mozambique close to the border with Tanzania, fide Muratov, 2010, and Van Bruggen, 2003), G. crux might represent a western outlier of the former. Van Bruggen (1969: 49) writes that “G. perissodonta favours a comparatively dry climate with a sparse vegetation; indeed it may be considered a typical savanna dweller.” Later, Herbert & Kilburn (2004: 185) explicitly state that G. perissodonta “occurs in drier habitats, particularly savanna woodland mosaic, in leaf litter and under stones and logs”. Ecologically, therefore, occurrence in fairly dry savanna woodland type vegetation from sea level to <1000 m, and in the wet montane forests of the Misuku Hills at c. 1800 m, does not seem to be compatible.
The conclusion here is that the shell under discussion is close to that of G. perissodonta, but does show sufficient differences for it to be considered to represent another taxon, here called G. crux. In addition, the above discussed differences in ecology point in the direction of the latter being a separate entity.