Mesodon thyroidus

Mesodon (Mesodon) thyroidus (Say, 1817)
White-lip Globe

  • Helix thyroidus Say 1817a: unpaginated.
  • ? Helix bucculenta Gould 1848 (1846–1850): 40.
  • Mesodon leucodon “Rafinesque” W. G. Binney & Bland 1870: 294, fig. 12.
  • Helix thyroides [sic] var. pulchella Cockerell 1892: 39, non Helix pulchella O.F. Müller, 1774.
  • ? Polygyra thyroides [sic] var. sanctisimonis Pilsbry 1901: 8.
Mesodon thyroidus; Pelee Island, Ontario (RGF 757). The parietal denticle is lacking in this shell.

Identification. Shell depressed-globose heliciform. Spire moderately elevated, conical. rather thin-shelled, a somewhat translucent. Whorls convex, evenly increasing in width. Spire: moderately elevated, conic. Suture: moderately indented. Last whorl: slightly shouldered; not descending towards the peristome; constriction of whorl behind the baso-palatal lip very weak; crest negligible. Periphery rounded, medial on last whorl. Teleoconch sculpture: narrow, close, and evenly spaced and sized colabral threads, crossed by strong microscopic spiral striae. Periostracum non-hirsute. Umbilicus narrow, ca ½ occluded by the columellar lip. Aperture subovate, wider than high. Apertural dentition ± triangular denticle on parietal, frequently weak or absent; no denticles on baso-columellar or palatal lips. Peristome, viewed from side prosocline, nearly straight. Parietal callus glazed, transparent, inconspicuous. Shell rather thin; silky lustre; pale straw-yellow; expanded lip and parietal denticle white. Shell to 31 mm wide (wider than high).

Animal greyish or yellowish white with darker ocular tentacles, and the sole of foot is dirty white (Baker 1939).

Habitat. In deciduous forests.

Canadian range. Ontario: almost entirely restricted to the two main southern Palaeozoic regions (Oughton 1948), but no recent records known east of Frontenac County. Historical record from Casselman (United Counties of Prescott and Russell).

Etymology. Unclear; perhaps Greek, thyreos, “a large, oblong shield” or thyra, “door”, + the suffix eidos, “form” or “likeness”.

Remarks. Mesodon thyroidus was originally described by Say (1817c) in his article “Conchology” that appeared in volume 2 of American Edition of the British Encyclopedia. Although usually dated 1816, this part was published in 1817 (Johnson 1975). The species name is sometimes misspelled thyroides.

Pilsbry (1940) also recognized a subspecies from Georgia, M. thyroidus sanctisimonis Pilsbry, 1901), which Hubricht (1985) regarded as a synonym of M. thyroidus. If subspecies are warranted, Canadian populations would belong to M. thyroidus thyroidus.