Gulella nuchalis spec. nov.
(figs 14-15, 16-18)
Diagnosis.— A species of Gulella s.l. characterized by a medium-sized (about 7 mm), costulate shell with apical spiral sculpture and with six-fold apertural dentition consisting of angular lamella, a weakly two-pronged labral complex, a mid-basal denticle, two outer and one inner columellar processes; behind the labrum of the shell there is a process that might be interpreted as the remains of a former labrum.
Description.— Shell (figs 14-15, 16-18) medium-sized (about 7 mm), subcylindrical, greatest width about the middle of the shell, about twice as long as wide, transparent when fresh to whitish when worn. Umbilicus slightly open and slit-like, shell therefore rimate. Spire produced, sides parallel to at most subparallel, not convex, apex flattened, obtusely conical to mamillate, with clearly marked fine spiral sculpture (fig. 17). Whorls from slightly more than six to somewhat less than seven, more or less convex, finely costulate (sometimes major and minor riblets may be distinguished), remains of spiral sculpture visible between the costulae. Sutures well impressed, subcrenellate to filiform. Aperture squarish to subovate, always higher than wide, with well reflected peristome, somewhat obstructed by six-fold dentition, consisting of a strong inrunning and perpendicular angular lamella, a
large, fairly superficial weakly two-pronged labral complex (corresponding to a noticeable pit behind the labrum), forming a large sinus with the angular lamella, a fairly deeply situated mid-basal denticle, two more superficial outer columellar denticles (one of which may also be considered to be left-basal), and a strong, mamillate inner columellar process. A short distance behind the labrum in the umbilical region there is what superfially looks like an old reflected labrum (figs 15, 18); this hump does not correspond to an inside depression. Juveniles do not show apertural dentition.
Measurements of shell: 6.4-7.4 × 3.1-3.4 mm, l/d 1.96-2.27, length last whorl 3.4-3.9 mm, aperture height × width 2.3-2.7 × 1.9-2.2 mm, whorls 6+-<7 (see table 3).
Table 3. Metric data of nine shells of the type lot of Gulella nuchalis n. sp. (all from Matipa-Wilindi Ridge, MCZ 298196 and RMNH 131918). The holotype shell has been indicated by an *, all other shells are paratypes.
Distribution.— So far this taxon has only been recorded from the Matipa Forest in the Misuku Hills in northern Malaŵi.
Ecology.— G. nuchalis n. sp. has only been reported from leaf litter in montane forest at c. 2000 m.
Derivatio nominis.— nuchalis, a Latin adjective of Arabian descent = pertaining to the nape or neck, referring to the peculiar structure shown behind the labrum of the shell.
Discussion.— All sorts of combinations of characters in Gulella s.l. keep on turning up all over Africa. The peculiar structure behind the labrum in G. nuchalis n. sp. probably has not yet been recorded. Of course, it is the combination of apertural dentition characters and apical spiral sculpture with this structure that makes the taxon under discussion seemingly unique (for a general discussion on shell characters in Gulella s.l. refer to Van Bruggen & Van Goethem, 1997: 7) and therefore a hitherto undescribed taxon. This outside hump does not correspond to an inside depression, pit or whatever.
Pilsbry (1919) is one of the few authors who supplies illustrations of some species of Gulella from the side, i.e. showing the shell laterally in addition to a ventral view. In some cases these figures give an indication of the phenomenon described above, but never as pronounced as in G. nuchalis. He calls this a “wave” on p. 221 (with reference to fig. 89b of Gulella disseminata cymatonotus Pilsbry, 1919): “the last [whorl] tapering downward, flattened dorsally, having a strong rounded wave on the right side and base some distance behind the lip, which is preceded by a rather broad contraction”. Lower down on the same page he writes: “Neither of Preston’s descriptions [of G. disseminata and G. ingeziensis] mentions the broad swelling or wave on the back, preceding the contraction behind the peristome.” Pilsbry’s fig. 91b of G. lessensis Pilsbry, 1919, on p. 222, also shows this phenomenon, albeit less marked, and he states on p. 223: “the last [whorl] tapering downwards, slightly contracted behind the lip, a low wave parallel to the lip preceding the contraction.” There are probably other cases in the literature where this particular ‘wave’ has been recorded.
The apertural dentition formula according to Verdcourt is either 1; 1; 1; 3 or 1; 1; 2; 2, depending on whether one process is interpreted to be a left-basal rather than an outer columellar process. The labral complex is weakly two-pronged and may be interpreted as the merger of two processes, so that the formula reads as follows: 1; 2; 1; 3 or 1; 2; 2; 2, resulting in a seven-fold apertural dentition. In addition, there may be an additional cusp on the right side of this complex (see fig. 16) so that the formula then is: 1; 3; 1; 3 or 1; 3; 2; 2, adding another process. The shell may then be considered having a eight-fold apertural dentition.
As regards similar shells one should check on taxa with a costulate shell in the size range of 6 to 8 mm with apical spiral sculpture and six or seven-fold apertural dentition. It appears that there are no described taxa with this combination of characters in southern Africa, Mozambique, Malaŵi, Zambia, East Africa and the D.R. Congo.
Details of the type locality are described in Van Bruggen, 1990 (pp. 100-101). The new taxon is unlikely to be a Malaŵi endemic – indeed, species described from southern Tanzania have been found in the Misuku Hills [e.g., Gulella cruciata (Von Martens, 1900), vide Van Bruggen, 2010].