Zilchogyra franzi Weyrauch, 1965

Delhey, Valdemar K., Pizá, Julia & Burela, Silvana, 2010, Zilchogyra franzi Weyrauch, 1965 (Gastropoda: Charopidae), a minute landsnail rediscovered in Sierra de la Ventana (Southern Pampas, Argentina), Zootaxa 2450, pp. 61-64 : 61-63

Zilchogyra franzi Weyrauch, 1965 b

Type locality: Sierra de la Ventana, Buenos Aires province, by original designation ( Weyrauch, 1965b).

Diagnosis: Large Zilchogyra species with a discoidal, almost flat shell with 4.3–4.9 whorls. Major diameter 4.3–5.4 mm. Spire low, slightly protruded. Ribs widely spaced, 45–56 ribs on the last whorl. Umbilicus broad, 36–50% of shell width.

Redescription: Shell depressed, discoidal flat with 4.3 to 4.9 markedly convex, gradually expanding whorls with deeply channelled sutures; semitranslucent, thin and fragile, with glossy, brownish periostracum. Major diameter ranging from 4.3 to 5.4 mm, shell height from 1.6 to 2.6 mm. Spire almost flat, slightly protruded (5–20% of shell height); most shells with only the first whorls visible in lateral view. Degree of descent of last whorl ( Figure 4 : bd/cd in %) ranging from 20 to 40%. Protoconch with 1.4 to 1.8 whorls, smooth, sometimes with rudimentary axial ribs in its distal portion ( Fig. 7 ). Teleoconch with axial ribs and intercostal microsculpture consisting of regularly spaced axial striae and weaker spiral lines, visible only under strong magnification ( Fig. 8 ). Axial ribs curved and retractive, distally with translucent periostracal projections, observable only in well preserved shells. Last whorl with 45 to 56 ribs (3.1 to 4.4 ribs per mm) and 6 to 14 axial striae in each intercostal space. Aperture subcircular, little excised by the last whorl. Peristome sharp, not expanded. Umbilicus wide, extending over about 36–50% of the shell width. Animal: Body colour greyish and dotted.

Discussion: Zilchogyra contains about ten species with broad South American distributions, inhabiting ecologically distinct regions, from Brazilian subtropical rainforests to the Patagonian Andes and Isla de los Estados (State Island). There are only two extant species of native Charopidae in the Pampas, Zilchogyra costellata (d’Orbigny, 1835) and Z. franzi . The classification of Zilchogyra and allied genera was recently discussed by Hausdorf (2005) and Miquel et al. (2007). Hausdorf tentatively defined Zilchogyra on the basis of anatomical and radula characters described by Hylton- Scott (1964) for the type species Z. costellata . However, anatomical data are lacking or insufficiently known for most South American punctoids. According to Miquel et al. (2007), Zilchogyra includes those charopids with smooth or almost smooth protoconchs, and differs from Lilloiconcha Weyrauch, 1965 in the structure of the axial ribs, being thick and almost straight in dorsal view in Lilloiconcha, and comparatively thin and curved in Zilchogyra .

The holotype of Z. franzi is smaller than 84% of our specimens (holotype measurements: major diameter 3.7 mm; minor diameter 3.2 mm; height 1.7 mm; whorl number 4.2). It also lacks characteristics of adult shells, such as a markedly descending last whorl. Thus the original description was probably based on a juvenile. However, the holotype is similar to specimens of comparable size in our material, both in shell shape and sculpture.

Zilchogyra franzi is conchologically close to Z. costellata , the type species. Weyrauch (1965b) noted that Z. franzi was smaller, with a wider umbilicus, a flatter spire, a more descending last whorl, a more circular aperture, and a higher number of ribs per mm. The size ranges of both species are similar. The other differences mentioned by Weyrauch agree with our data, the main character separating both species being the degree of protrusion of the spire.

Figures

FIGURES 4 – 6. Scanning electron micrographs of Zilchogyra franzi showing shell measurements. 4, dorsal view, black line: major diameter; 5, ventral view, black line: umbilicus width; 6, frontal view. ab, spire protrusion; bc, body whorl width; ad, shell height.

FIGURE 7, 8. Scanning electron micrographs of Zilchogyra franzi. 7, protoconch; 8, the axial ribs and the intercostal microsculpture.