Bensonella plicidens

Bensonella plicidens (Benson, 1849)

(Figures 3 (a–e), 4, 5, 9(a,b)).

Pupa plicidens Benson, 1849: 126

Pupa plicidens – Küster 1841 –1855: 136, pl. 17, figs 23–24

Boysidia plicidens – Pfeiffer 1853: 553

Vertigo (Odontocyclas) plicidens – Adams and Adams 1858: 173

Pupa (Scopelophila) plicidens – Albers and Martens, 1860: 296

Boysidia plicidens – Hanley and Theobald, 1874 (1870–1876): pl. 100, fig. 8

Boysidia plicidens – Sowerby, 1876: pl. 16, fig. 151

Pupa (Odontocyclas) plicidens – Pfeiffer, 1879 (1878–1881): 350

Boysidia plicidens – Ancey 1881: 373

Bifidaria (Bensonella) plicidens – Pilsbry and Vanatta 1900: 591

Boysidia plicidens – Gude 1914: 294

Bifidaria (Bensonella) landurensis [sic] Pilsbry 1915: 73–74 new synonym

Boysidia landurensis – Pilsbry, 1917 (in Pilsbry 1916 –1918): 204, pl. 35, fig. 9

Paraboysidia landourensis – Benthem Jutting, 1950: 39

Bensonella plicidens – Hwang 2014: fig. 5 (partim: fig. 5 shows B. plicidens)

Bensonella landourensis and plicidens – Budha and Backeljau 2017 (partim: figs 2B, 3, 5 A and F show B. plicidens)

Types examined

UMZC I.103325.A, lectotype of P. plicidens (selected herein from a series of 15 syntypes in the UMZC), R. McAndrew coll. (ex- W.H. Benson coll.), Rockville, Landour, Himalaya; NHMUK 1954.6.2.937, paralectotype of P. plicidens, J.S. Hawkins coll. (ex- W.H. Benson coll.), W Himalaya (labelled as paratype); ANSP 16721, holotype of B. landourensis (photos examined: see Figure 5).

Additional material examined

NHMUK 1856.9.15.69/A, 36 shells, Capt. T. Hutton coll., Moussorie; NHMUK 20191111/A, 10 shells, Cherra, India, mixed sample that includes NHMUK 20191111/B, identified as B. hooki sp. nov.; NHMUK 1903.7.1.2882/B, six shells, Godwin-Austen coll., Moussorie, NW Himalaya, mixed sample that includes NHMUK 1903.7.1.2882/C, identified as B. hooki sp. nov.; NHMUK 1906.2.2.190/B, 7 shells, W. T. Blanford coll., Cherra Poonje, Khasi Hills, mixed sample that includes NHMUK 1906.2.2.190/C, identified as B. hooki sp. nov.

Extended diagnosis

Shell triangular (pyramidal), slightly ovoid, height somewhat variable; shell colour greyishyellowish and corneous; sculpture with inconspicuous growth lines and very fine spiral striation (its strength variable, in some shells nearly absent); aperture with numerous barriers; parietal wall with a parietal and an angular lamella, both being lower at their middles, and a third, short, deeply situated one between parietal lamella and columellarparietal junction; palatal wall with three long main plicae with elevated central parts, usually (but not always) with additional short plicae between them; columellar lamella similar to palatal plicae; between the lowest palatal plicae and the columellar lamella there are two short plicae; an additional, blunt tubercle sits on the edge of the palatal lip, at the position where the palatal lip forms a little sinus; lower part of columellar lip with two very low, blunt denticles (swellings).

Differential diagnosis

For differences with B. hooki sp. nov., see under that species. Bensonella plicidens differs from B. lakainguta in the following traits: shell shape is similar, but B. plicidens is wider triangular; the apex is less pointed; the sculpture is clearly densely spirally striated, whereas B. lakainguta is smooth; the arrangement of the apertural barriers is similar, but the barriers of B. plicidens are elongated ridges, which are highest at their middle parts, whereas in B. lakainguta the central part of each barrier is a hook pointing anteriorly (outside of the shell), with an anteriorly elongated low ridge before the hook.

Remarks

The original spelling of B. landourensis was B. landurensis by Pilsbry (1915), although the type locality was spelled Landour (town in Mussoorie, India). Therefore, the spelling ̍ landourensis ̾ by van Benthem Jutting (1950) is a justified emendation, and ̍ landurensis ̾ is an incorrect original spelling (under ICZN Art. 32.5).

Pilsbry (1915) distinguished B. landourensis from B. plicidens by the uninterrupted angular lamella in the former. Additional differences were also mentioned, such as the length of the angular lamella, which does not emerge to the edge of the peristome (i.e. it stops before the peristome edge) in B. landourensis but does in B. plicidens, and the parietal lamella, which is shorter in B. plicidens than in B. landourensis. Although the original description of B. landourensis does not mention hooked apertural barriers in B. plicidens, it is clear that Pilsbry̾s (1915) new species was not compared with correctly identified B. plicidens but with the species that is described here as new (B. hooki sp. nov.), because B. hooki sp. nov. has an interrupted angular lamella, whereas that of B. plicidens is not interrupted. Comparison of the holotype of B. landourensis with syntypes of B. plicidens reveal no significant differences, other than the somewhat more slender shell of B. landourensis, which can be explained by intraspecific variability.

Pilsbry (1915) mentioned that the single shell of B. landourensis (= B. plicidens in our current understanding) was found amongst shells of B. plicidens (= B. hooki sp. nov. in our current understanding), all of which had been received from Benson. We were unable to examine the shells studied by Pilsbry, other than the holotype of B. landourensis. It is possible that Benson had both B. plicidens (a species with normal barriers) and B. hooki sp. nov. (a species with hooked barriers) when he described B. plicidens. However, neither Benson (1849) nor subsequent authors mentioned hooked barriers as a character of that species, and the syntypes in the original Benson collection in the UMZC all have normal barriers (Preece et al., in litteris). Even the name ̍ plicidens ̾ indicates that the apertural barriers were plica-like.

We do not know how Pilsbry received Benson̾s original specimens, or whether it is really true that he did. It is remarkable that all of the syntypes of B. plicidens that remain in England (UMZC and the shell from the J.S. Hawkins collection in the NHM) have normal barriers, but the lot Pilsbry received, according to this analysis, was entirely hooked with only one normal B. plicidens that was redescribed as B. landourensis by Pilsbry (1915). It is possible that the ̍ B. plicidens ̾ specimens that Pilsbry (1915) referred to as originating from Benson were not part of the type series, or were not directly from William Benson.

Here we select the syntype in Cambridge (UMZC) as a lectotype to avoid further nomenclatural problems. Consequently, B. plicidens is a species with normal apertural barriers and B. hooki sp. nov. is a species with hooked barriers.