Dyakia janus (Beck, 1837)
Remarks on Helix balestieriana – “Any species of the Gasteropoda with the sinistral character, however inobtrusive it may be in other respects, must always prove highly interesting. I owe this, with many other fine shells, to the kindness of Mr. Balestier, of Singapore. It is, perhaps, more nearly allied to H. himalana, (nobis,) than to any other species, but may at once be distinguished by its thickened and reflected lip; also by its darker colour, and having two more whorls, which are less flat. It is also a larger shell, but not so large as H. senegalensis, Lam., (which comes here from Canton in abundance,) the latter being a more globose species, with transverse lines.” (Lea, 1848)
“Diese Art unterselieidet sich durch ihre zarte Sehale, viel feinere Streifung, Haches, dichtes Gewinde, sehr scharfen Kiel und einfachen Mundsaum von N. amphidroma; doch ist es bei einzelnen unausgewachsenen Exemplaren oft schwer, sich für die eine oder andere Art zu entscheiden, da auch N. janus etwas in der Erhebung; des Gewindes variirt. Nach Vergleichung von Text und Figur glaube ich bestimmt, dass H. janus von Chemnitz hieher gehört, das Weiss der Oberseite beruht dann übrigens auf Verbleichung der todten Schale, wäe mir selbst solche halbseitig ausgebleichten Exemplare von Singkawang vorliegen. Auch der regalis kommt unsere Art sehr nahe, bleibt aber stets flacher und ihre Sculptur schwächer, ihre Färbung gleichmässig. Chemnitz nannte sie janus, weil die Färbung der Oberseite von der der Unterseite bei seinem Exemplar so sehr abwich; das war, vermuthe ich, nur zufällig, aber man kann dem Namen jetzt die andere Bedeutung unterlegen, dass die Art als solche ihr Gesicht sowohl nach der rechten als nach der linken Seite wendet, freilich das einzelne Individuum nur nach einer.” (Martens, 1867)
Lea, I. (1841) original descriptions on Helix balestieriana – “Testâ orbiculato-conoideâ, sinistrorsâ, subcarinatâ, subtenui, castaneâ, infernè, inflatâ, prope periphlsriam tenebrosiori, minutè rugoso-striatâ, perforatâ; anfractibus senis, convexis; aperture parvâ; labro incrassato, subreflexo; columellâ levi.”
Lea, I. (1841) English descriptions on Helix balestieriana – “Shell orbicular conoidal, sinistral, somewhat carinate, rather thin, chestnut-coloured, inflated below, darker near the periphery, with minute rough striae, perforate; whorls six, convex; aperture rather small, lip thickened, slightly reflected; columella smooth.”
Pfeiffer (1848) descriptions on Helix janus – “T. perforate, sinistrorsa, orbicularis, tenuis, diaphana, oblique regulariter et confertim costulato-striata, superne fusca, basi convexa rufo-castanea, nitidiuscula; spira late conoideo-epressa; anfr. 7 planulati, ultimus carinatus; aperture lunaris; perist. tenue, acutum, margine columellari reflexiusculo. – R.a. = 4:5.”
Eydoux & Souleyet (1852) descriptions on Helix mackensiana – “Helix, testâ discoideâ, sinistrorsâ, umbilicatâ, carinatâ, supra depresso-conicâ et pallidè fulvâ, infrâ convexâ et castaneâ; anfractibus septenis, planiusculis, transvertim tenuissimè striatis; aperture semi-ovatâ, anticè sub-acutâ; peristomate sub-reflexo, albo; umbilico profundo.”
Eydoux & Souleyet (1852) French descriptions on Helix mackensiana – “Coquïlle discoide, sénestre, ombiliquée, carénée à la circonférence, conique-déprimée et de couleur fauve en dessus, convexe et de couleur marron à sa face inférieure. Sept tours de spire un peu planes et marqués de stries transversals, fines et serrées. Ouverture demiovale, Presque aiguë en avant; péristome un peu réfléchi, blanc. Ombilic étroit et profound. Nous n’avons pas observé l’animal.”
Martens (1867) descriptions on Nanina janus – “Testa modo dextra, modo sinistra, perforata, depressa, acute carinata, supra striis obliquis laevibus subdistantibus arcuatis et lineis subtilissimis spiralibus confertissimis decussantibus sculpta, rufescens vel brunnea, opaca, infra confertius striatula, pallidior, nitida, carina saepius albida; spira depresse conica; anfr. 6½, plani, sutura lineari, superficiali, ultimus infra multo convexior, antice haud desceudens; apertura diagonalis, securiformis; peristoma simplex, acutum, tenue, margine basali valde arcuato, columellari ad insertionem breviter reflexo.”
Martens (1867) descriptions on Nanina amphidroma – “Testa modo dextra modo sinistra, aperte perforata, conico-depressa, angulata; superne striis subobliquis confertis argutis plus minusve granulosis sculpta, opaca; inferne striis laevibus multo subtilioribus sublaevigata, nitidula, castaneo-rufa vel pallide brunnea; spira breviter conica, apice subtilius striata; anfr. 7, lente crescentes, fere plani, ultimus distincte angulatus, infra convexus, antice non descendens; apertura obliqua, semielliptica, peristoma album, crassiusculum. margine supero antrorsum angulatim producto, basali toto expanso, antrorsum flexuoso. A) martini Pfr.: Major, magis depressa, obtuse angulata, striis granulosis, superne cinnamomea, inferne obscurius castanea, peristomate toto breviter expanso. B) intermedia: Forma et colore priori similis. carina magis distincta, margine supero vix expanso. C) mackensiana Souleyet: Minor, magis elata, carinata. striis vix granulosis, superne et inferne concolor, pallide brunnea, margine supero recto.”
Laidlaw (1931) descriptions on Dyakia janus – “Shell sinistral, depressed trochiform, narrowly perforate, rather thin, horn coloured rather paler above. Whorls 6½, increasing slowly; suture not much impressed, carinate, but the carina tends to become less marked near the aperture. Below the carina the shell is rather more inflated. Aperture sublunate, peristome slightly thickened and very slightly reflected, the margin of the peristome below the carina is sinuous. The surface is finely marked with very numerous, regular, closely-set, radial striae; these under strong magnification are distinctly granulose.”
Helix balestieriana – “Diam. 1.4, length 0.8 of an inch.” (Lea, 1841); Helix janus – “Diam. maj. 36, min. 31, alt. 17 mill.” (Pfeiffer, 1848); Helix mackensiana – “Dimensions de la coquille – Largeur, trente millimètres; hauteur, quatorze millimètres.” (Eydoux & Souleyet, 1852); Nanina janus – “Diam. maj. 33, min. 29, alt. 17; apert. long. 16½, lat. 11 Mill.” (Martens, 1867); Nanina amphidroma form martini – “Diam. maj. 27½-43, min. 22-34, alt. 14-26; apert. long. 19-25, alt. 14 1/3-19 Mill.” (Martens, 1867); Nanina amphidroma form intermedia – “Diam. maj. 39, min. 32½, alt. 23; apert. long. 21½, alt. 17 Mill.” (Martens, 1867); Nanina amphidroma form mackensiana – “Diam. maj. 35-37, min. 21½-31, alt. 16-24; apert. long. 15-21, alt. ? Mill.” (Martens, 1867); Dyakia janus – “Diam. max. 30 mm., alt. 16 mm.” (Laidlaw, 1931)
Type locality – Helix balestieriana “Philippine Islands” leg. Balestier (Lea, 1841); Dyakia mackensiana “Malacca” (Souleyet, 1841)
Other localities – Helix janus “ monte Ophir peninsulae Malaccanae” (Pfeiffer, 1848); “Westküste von Borneo: bei Singkawang” (Martens, 1867); “Westküste von Borneo: bei Lumar” (Martens, 1867); “Westküste von Borneo: bei Mandhor” (Martens, 1867); “Singapore” (Martens, 1867); “Bukit Besar, Nawngchik, 2500 feet” (Collinge, 1903); Dyakia mackensiana “Perak” (Laidlaw, 1931); “Sungai Ujung” coll. Laidlaw (Laidlaw, 1931)
Distributions – “Malakka, Sumatra und Borneo” (Martens, 1867)