Helix decidua Pfeiffer, 1857

Brook, Fred J. & Ablett, Jonathan D., 2019, Type material of land snails (Mollusca: Gastropoda) described from New Zealand by taxonomists in Europe and North America between 1830 and 1934, and the history of research on the New Zealand land snail fauna from 1824 to 1917, Zootaxa 4697 (1), pp. 1-117 : 96-97

Helix decidua Pfeiffer, 1857

Pfeiffer, 1857. Proceedings of the Zoological Society of London, 25: 108.

Type material: Original description based on one or more specimens in the Cuming collection; type material not found in NHMUK collections in 2018, whereabouts unknown .

Type locality: ‘New Zealand’ ( Pfeiffer 1857b: 108).

Remarks: There are no published illustrations of the type material. Hutton (1884b: 203) and subsequent authors have generally included this species in the genus Therasia Hutton, 1883 (e.g., Pilsbry 1892 [in 1892–1893]: 71; Hedley & Suter 1893: 641; Suter 1894d: 245, 1904a: 63, 1913b: 657; Powell 1979: 316; Spencer et al. 2009: 216), but this appears to be based on a misinterpretation. Pfeiffer’s original description referred to a ‘depressed turbiniform’ shell, of five whorls, that was 3.5 mm wide and 2 mm high. By contrast, descriptions and illustrations by later authors (e.g., Hutton 1884b: 203; Pilsbry 1892 [in 1892–1893]: 71, pl. 22, figs. 52–54; Suter 1913b: 657, pl. 50, fig. 8; Powell 1979: 316) referred to shells that were much larger and more loosely-coiled, with diameters of c.8.0–9.5 mm, and which were almost certainly not conspecific with Helix decidua Pfeiffer. In the absence of the type material it is not possible to reliably determine the identity of Pfeiffer’s species, but the original description suggests that it is probably a species of Laoma Gray, 1850 or Phrixgnathus Hutton, 1883 , sensu Suter (1913b) and Powell (1979) [ Punctidae ].

Systematic position: Not determined.

Distribution: Probably native to New Zealand, but regional distribution not known.