Labyrinthus Beck, 1837

Cuezzo, Maria Gabriela, 2006, On a new species of Isomeria Beck and redescription of species of Labyrinthus Beck from South America (Gastropoda: Stylommatophora: Camaenidae), Zootaxa 1221, pp. 1-23 : 3-4

Genus Labyrinthus Beck, 1837

Type species: Helix otis Lightfoot, 1786 (= Helix labyrinthus "Chemnitz " Deshayes, 1838) by subsequent designation by Herrmannsen (1846: 569).

Diagnosis: Shell small to large (10–60 mm) usually uniformly colored; sculpture of wrinkles or granulations, without axial ribs; periphery angulated or keeled; aperture strongly obstructed with denticles and lamellae; shell mark of basal internal tooth always diagonal respect to apertural lip; parietal single lamellae rectangular, merged or not with parietal lip; parietal lip usually raised, lower inner internal palatal lip wall with T­shaped lamellae, or two Y­shaped teeth or conical transverse lamellae; jaw smooth or weakly ribbed; alveoli of ovotestis ovoid to digitiform; bursa copulatrix oval, well differentiated, reflexed over distal portion of duct; penis (in some species also vagina) with hooked denticles of 300 to 400 m of basal plate maximum length.

Distribution: From Costa Rica south to the Cuzco department in Peru. Easternmost records are in Pará Brazil, although Labyrinthus is mainly known from Colombia, Ecuador and Peru.

Discussion: Labyrinthus is a monophyletic genus, characterized by the following synapomorphies ( Cuezzo 2003): basal internal tooth with diagonal shell mark; internal parietal teeth rectangular or folded merging with the peristome; lower internal palatal wall with two Y­shaped teeth; body whorl shouldered; ovotestis with digitiform alveoli and bursa copulatrix sac reflexed over the duct.

The presence of the reflected penis­epiphallus attached by muscular strands is a character shared with its closest relatives Isomeria and Solaropsis . Internal hooked denticles are present on top of the ridges in penis and vagina in the two investigated species of Isomeria but not in the eight species of Solaropsis , in which anatomy was studied ( Tillier 1980, Cuezzo 2001). Penial hooked denticles have only been reported in the Scolodontidae (= Systrophiidae ), in Streptaxidae as well as in some Zonitidae and Oxychilidae in the Stylommatophora ( Hausdorf 2003) . Similar hooked denticles were reported in dorid nudibranchs ( Valdés 2004), in which they function to reinforce the anchoring of the penis inside the vagina of the partner to prevent premature separation ( Valdés 2004). However, in both Labyrinthus and Isomeria the hooked denticles are present not only in the penis but also in the vagina in most of the species. The function of the penial and vaginal hooked denticles in both genera is probably related to sexual stimulation and to anchoring penis in the vagina. Penial denticles are smaller in Isomeria (180–200 m of basal plate length) than in Labyrinthus (300–400 m of basal plate length).