Comments on “A new species of land snail from the genus Diplommatina Benson, 1849 (Gastropoda, Caenogastropoda, Diplommatinidae) from Sikkim Himalaya, North East India” by N. K. Das & N. A. Aravind, Molluscan Research 41 (3), 262–268, 2021

ABSTRACT

Corrigendum to the paper, ‘A new species of land snail from the genus Diplommatina Benson, 1849 (Gastropoda, Caenogastropoda, Diplommatinidae) from Sikkim Himalaya, North East India’ published in Molluscan Research 41, 1–7, 2021 DOI: 10.1080/13235818.2021.1970352.

KEYWORDS:

• Carychiidae
• Carychium
• corrigendum
• Ellobioidea
• Eupulmonata
• Himalayan species
• microsnail
• systematics
• taxonomic acts

Diplommatina Benson, 1849 is an Asiatic genus encompassing more than 400 species (MolluscaBase, 2021). Although Benson (1849) initially erroneously placed Diplommatina within the ellobioid family Carychiadae (= Carychiidae), he overlooked the presence of an operculum, which later resulted in its being transferred into the operculate Cyclophoridae (Caenogastropoda) by Gray (1850).

Diplommatina (Cyclophoroidea, Diplommatinidae) and Carychium O. F. Müller, 1773 (Eupulmonata, Ellobioidea, Carychiidae) occupy the same layer of moist forest leaf litter in their Asian biomes (AJ pers. observ.) and mistaking one for another is not a remote possibility. Although Carychium is frequently radially ribbed, the degree of ribbing varies within the genus and ribbing is not a constant morphological character in all species. The shell surface of Diplommatina is almost always ornamented with widely or narrowly spaced radial ribs and a continuous, circular aperture mostly bearing a double peristome. Carychium rarely bears a round peristome and its aperture is oblong in shape. The shell of Diplommatina shows a constriction at the ventral side of the body whorl, causing a tightening or squeezing effect on the corresponding surface (Figure 1E, F). The degree of constriction corresponds to the diameter of the operculum (Neubert and Bouchet 2015). In addition, most species have a palatal lamella developed in front of the constriction, with a parietal lamella and a columellar lamella present.

Figure 1. A, ‘Diplommatina’ parietidentata Das & Aravind, 2021, from original description; B, Carychium khasiacum Godwin-Austen 1877, Nongba, India, NHMUK 1903.7.1.2850, syntype; C, Carychium minimum O.F. Müller, 1774, France, MNHN-IM-2010-13017; D, Inner structure of Carychium minimum O.F. Müller, 1774, France, MNHN-IM-2010-13017; E, F, Diplommatina paxillus (Gredler, 1881), Hubei, China; G, Diplommatina scolops Mollendorff, 1901, Guangxi, China; H, Inner structure of Diplommatina sp., Zhejiang, China. Scales all 1 mm. Photos: Kevin Webb (B, ©NHMUK), Olivier Gargominy (C–D, ©MNHN), Zhe-Yu Chen (E–H).

In their recent publication, Das and Aravind (2021) described a new species of Diplommatina from Sikkim, diagnosed by bearing ‘a curved, single peristome, and a uniquely identifiable feature of a distinct, external prominent parietal tooth on the parietal peristome of shell aperture’. Although this work is highly descriptive concerning the northeast Indian Diplommatinidae, it is odd that diagnoses of Diplommatina by Benson (1849), Gray (1850) and Kobelt (1902) are not considered although the authors repeatedly emphasise that their new species, Diplommatina parietidentata Das & Aravind, 2021, is significantly different from all other members of Diplommatina. Moreover, Das and Aravind (2021) cited the work of Budha et al. (2017) but overlooked the significant Benson-Gray taxonomic scenario underscored by those authors. By failing to recognise that Diplommatina parietidentata is actually Carychium, Benson’s (1849; 1853) pitfall was revisited, albeit conversely, in this newest work.

Like its congeners from North and Central American, Eurasian, Asian and SE Asian biomes, Carychium from the Himalayas (Raheem et al. 2010; Gittenberger et al. 2017) and the Tibetan plateau (AJ unpubl. data) is known to inhabit moist forest habitats. Species from India include Carychium indicum Benson, 1849, Carychium khasiacum Godwin-Austen 1877 and a dubious species described from a juvenile shell (most probably C. indicum) named Carychium boysianum Benson, 1864.

Based on apparent shell characters here, Das’ and Aravind’s new species, D. parietidentata, belongs to the ellobioid genus, Carychium and not the cyclophoroid genus, Diplommatina due to the following morphological reasons. The shell is oblong or turreted to cylindric in form, and the aperture is oblong-ovate and not round as that characteristic for the Diplommatinidae (Figure 1). Carychium usually bears a low discrete lamella (or thickening as in D. parietidentata) near the base of the columella and a more or less prominent, parietal lamella above, which expands within the last whorl. Although not directly visible in D. parietidentata here, it can be expected that the internal partitions and axis are absorbed in the upper whorls, made visible here via the cutout shell of Carychium minimum O. F. Müller, 1773–1774 in Figure 1D. For additional information concerning shell characters of Diplommatina species of the Himalayan region, please consult Budha et al. (2017). Lastly, until available molecular data can be integratively assessed with all known Carychium shell material from the Himalayan region, which is planned in an upcoming revision of the genus, regarding D. parietidentata as a distinct species would be taxonomically premature. We strongly encourage the authors, Das and Aravind, to contribute preserved D. parietidentata material for the molecular assessment of this Himalayan member of the genus.

Disclosure statement

No potential conflict of interest was reported by the author(s).