Hystricella aucta (Wollaston, 1878) Wollaston, 1878

Mattia, Willy De, Neiber, Marco T. & Groh, Klaus, 2018, Revision of the genus-group Hystricella R. T. Lowe, 1855 from Porto Santo (Madeira Archipelago), with descriptions of new recent and fossil taxa (Gastropoda, Helicoidea, Geomitridae), ZooKeys 732, pp. 1-125 : 14-17

† Hystricella aucta (Wollaston, 1878) stat. n. Figs 71-73, 75

List of synonyms.

1867 Helix (Octephila) vermetiformis var. α minor Paiva: 48 [non Helix minor mult. auct.].

1878 Helix (Hystricella) bicarinata var. ss aucta Wollaston: 162-163.

1950 Discula (Hystricella) bicarinata aucta - Mandahl-Barth: 31, 55.

1983 Discula (Hystricella) bicarinata aucta - Waldén: 267.

2002 Geomitra bicarinata aucta - Bank et al.: 124.

2006 Discula bicarinata aucta - Cameron et al.: 31.

2008 Hystricella bicarinata aucta - Seddon: 79.

2009 Hystricella bicarinata aucta - Groh et al.: 21 fig. 25.

Type material.

Despite intensive research in multiple museum collections (NHM, NMW, MMUE, ANSP, NHC, NMS, RAM, SMF) that could have held the type material of the taxon, no such material could be traced and therefore we deem it reasonable to assume that the type material is lost. To stabilise the present interpretation of Helix (Hystricella) bicarinata var. ss aucta Wollaston, 1878 and to clarify its taxonomic status we designate a neotype here, which is deposited in the collection of the SMF, under the No. SMF 348937 (see Fig. 71). The neotype is consistent with the original description (see below), especially with regard to size, shape and the presence and development of the two keels on the body whorl. The taxon was originally described from Porto Santo, which is consistent with the origin of the specimen selected as neotype.

Loci typici.

[aucta] Barbinha, Quaternary slope deposits, "grey layer", 33°04'04"N / 16°17'49"W, 8 m, (through the designation of the neotype); [var. minor]: … rara ad Zimbral d'Aréa.

Further material examined.

All from Porto Santo, CKG/1, S slope of Pico do Concelho, Quaternary slope deposit, 33°04'35"N / 16°18'03"W, 200-230 m, leg. K. & C. Groh, Jun. 29 1980; CKG/2, Barbinha, Quaternary slope deposits, "grey layer", 33°04'04"N / 16°17'49"W, 8 m, leg. K. & C. Groh & J. & C. Hemmen, Jul. 4 1983; ANSP H 11789/9 [sub H. bicarinata ], Barbinha, Quaternary slope deposits, "red layer", 33°04'04"N / 16°17'49"W, 8 m, leg. J. Gerber, K. Groh & J. Hemmen, Aug. 12 1985. CKG/1, first bay S of Barbinha, Quaternary slope deposit, 33°03'51"N / 16°17'46"W, 18 m, leg. K. & C. Groh & J. & C. Hemmen, Jul. 5 1983. CGK/3, Ponta da Canaveira, Quaternary aeolinites, 33°02'30"N / 16°23'35"W, 50 m, Jun. 24 1983, leg. K. Groh & J. Hemmen; CGK/3, S of Ponta da Canaveira, Quaternary aeolinites, 33°02'30"N 16°23'34"W, 55 m, Oct. 26 1980, leg. K. Groh & J. Hemmen; CKG/2, Vale do Touro, Quaternary slope deposit cut by water erosion in the banks of the valley, 33°03'48"N / 16°19'17"W, 15 m, leg. K. & C. Groh, Aug. 16 1980; CWDM/14, S slope of Vale do Touro hill, 50 m W of the oil tanks, fossils beds in mixed gravel and mud deposits, 33°03'47"N / 16°19'26"W, 15 m, leg. W. De Mattia & J. Macor, May 2015; CFW 12172/1 fragm. [cf. aucta ], Ponta da Galé, E end of tunnel, lower level [of slope deposits], coarse, red coloured gravel with large stones, 33°03'44"N / 16°17'45"W, 30 m, leg. F. Walther, Apr. 4 2017; CFW 12173/16, CKG/3, S of Ponta da Canaveira, (sub-)fossil [slope-]deposits, coarse, red coloured gravel, 33°02'25"N / 16°23'41"W, 30 m, leg. F. Walther, Apr. 2 2017; CFW 12175/8, E of Vila Baleira, end of Vale do Touro, (sub-)fossil [slope-] deposits, coarse, black gravel, 33°03'48"N / 16°19'15"W, 20 m, leg. F. Walther, Apr. 5 2017; CFW 12174/13, CKG/2, E of Vila Baleira, S slope of the hill above Vale do Touro, W of the oil tanks, [(sub-)fossil slope-deposits of] red gravel, 33°03'47"N / 16°19'26"W, 25 m, leg. F. Walther, Apr. 5 2017.

Original descriptions.

[aucta]: From Wollaston 1878: There is however an appreciably larger form of this species [bicarinata] (cited in the present catalogue as the 'var. ß. Aucta’) to which the subfossil examples might perhaps be better referred, - in which the upper (or medial) keel is a trifle more horizontal and prominent, and the shell is full 3 lines (instead of only approximately 2½) across its broadest part - which was found in Porto Santo by Mr. Watson, and which I have received from him as the 'recent state of the H. vermetiformis, Lowe.' the 'var. ß. Aucta' of the H. bicarinata in its very much larger size, and in its volutions (the ultimate one of which is not quite so deflected at the aperture) being 7 in number, instead of only 6 or 6½; [var. minor]: From Paiva 1867: Variet. adest α minor, testa minore.

Redescription of the shell.

Shell small for the genus, with 5.7 regularly increasing whorls, the protoconch with 1.9 whorls. The form of the shell is conical, the concave teleoconch whorls, with two prominent keels situated at the upper ⅐, respectively ⅖ of the total height of the last whorl. The last whorl measures 62%, the penultimate whorl 19% of the total shell height. The lower 60% of the body whorl beneath the second keel are rather straight, only a little convex in the middle. The suture is simple, only slightly sunken beneath the second keel of the preceding whorl. The aperture, which is inclined to the vertical axis of the shell in an angle of 49° and is descending in the last 5% of the last whorl in an angle of 38° to the horizontal axis, has an elliptical-ovate form, its width is 42% of the total shell width and its height 27% of the total shell height. It has an only slightly reflected lip, which is completely detached from the body whorl. The eccentric umbilicus, which is approximately 12% as wide as the shell, is in the upper whorls needle stitch-like. The protoconch is smooth. The teleoconch is equipped with few oblique radial ribs, seven in the penultimate quadrant of the body whorl and is additionally covered by numerous coarse tubercles. The number of tubercles in the standard quadrate of the base is 31; the tubercles are of approximately equal size. There are no traces of colouration (Figs 71-73).

Measurements.

D 5.5 ± 0.5 mm (range 5.1-6.4 mm); H 4.6 ± 0.4 mm (range 4.1-5.3 mm); FW 2.8 ± 0.2 mm; PA 51 ± 2.1; DU 0.5 ± 0.03 mm; NT 18 ± 6; NW 5.5 ± 0.4 (n = 22). Ratio D/H 1.2; ratio FW/H 0.6.

Distribution.

Hystricella aucta is known only from the southeastern coast of Porto Santo: from the hill immediately east of Vila Baleira (mud and slope deposits at Vale do Touro) to the mud deposits and aeolinites along the southeastern coast (Barbinha, Zimbral da Areia, and Porto dos Frades) (Fig. 75).

Comparison and comments.

Because of its small size, the species can only be confused with the similar sized recent H. bicarinata which, however, is somewhat larger on average, has less developed keels, especially the upper one is less prominent and has coarser and denser tubercles on its shell surface, a narrower umbilicus and a more distinctly descending aperture. The smaller H. microcarinata sp. n., which has a much more rounded globular form, bears less prominent keels on the whorls, has finer and more densely arranged tubercles, has a much narrower umbilicus and a different shape as well as differently positioned angles of the aperture. The similar sized Wollastonia beckmanni sp. n., which has a much flatter shape, has no keels on the whorls, a much wider umbilicus, much more and finer tubercles and differently positioned angles of the aperture.

Nomenclatural and taxonomic remarks.

Considered by Wollaston (1878) as the fossil variation of H. bicarinata . We here consider it as a distinct species because of the shell differences. Like H. bicarinata , H. aucta has previously ( Mandahl-Barth 1950, Waldén 1983, Bank et al. 2002) been listed as belonging to Discula and later on to Geomitra .

Status and conservation.

Extinct before the islands’ scientific exploration in the 19th century, possibly already before human settlement.