Sometimes small details can be overlooked. I did just this when preparing the description for Land Snails of British Columbia (Forsyth 2004).
In email discussion with the late Andrew Grebneff, who first queried this topic on Conch-L, it was brought to my attention that shells of Allogona ptychophora sometimes bear a small denticle or “tooth” on the parietal wall. That was back in 2007.
This denticle is small, low and in some shells little more than a whitish callus patch. But, another look at the literature today confirms that a parietal denticle was not seen by H. A. Pilsbry (1940). In his classic Land Molluscs of North America, he wrote “… [t]he rounded aperture has a reflected white peristome, its inner edge usually having a blunt tooth or low callus in the baso-columellar curve; [there are] no other teeth” (1940: 875). Nothing about a parietal denticle is mentioned later in the accounts for any species of Allogona, including A. ptychophora.
Among polygyrids, Allogona shells are relatively weakly dentate. However, given the diversity and convergences in shell form among members of this diverse family, it isn’t surprising, as Andrew Grebneff thought, that traces of “extra” denticles might appear.
It is useful to note that the key in Land Snails of British Columbia will lead to the wrong conclusion (i.e., Cryptomastix) if an Allogona ptychophora shell has the parietal denticle.
Lesson learned: check and recheck, and look at as many specimens as possible . . . details can be overlooked.
Revised key to Polygyridae in BC
1a Poorly to strongly developed parietal denticle present in adults (it may be only a small, opaque whitish callus) — 2
1b Parietal denticle altogether absent — 5
2a Parietal denticle generally rather well-developed. Periostracum hirsute in some — 3
2b Parietal denticle weak, just a slight callus. Periostracum never hirsute. (Presence of a parietal denticle is very occasional in this species.) — Allogona ptychophora, in part
3a Mature w < 9 mm. Palatal lip rather thin, strongly flared but not recurved. Parietal denticle, in basal view, long and curved. Periostracal hairs generally persistent in adults — Cryptomastix germana
3b Mature shell width ≥ 12 mm. Palatal lip thicker, expanded, and recurved. Parietal denticle short-elongate to tubercular. Periostracal hairs usually not persistent in adults; if they are present, then short and relatively sparsely spaced — 4
4a Mature shell width ≤ 26 mm, slightly depressed-globose. Historically southwest, coastal British Columbia — Cryptomastix devia
4b Mature w ≤ 19 mm, usually rather more depressed-globose. South and southeast interior of British Columbia — Cryptomastix mullani, in part
5a Periostracum hirsute (hairy), unless worn smooth or completely eroded off — 6
5b Periostracum not hirsute — 7
6a A few weak spiral striae present. Palatal lip recurved. Periostracal hairs densely spaced, rather long — Cryptomastix mullani, in part
6b Spiral striae absent. Palatal lip flared outward but not strongly recurved. Periostracal hairs densely spaced, rather long — Vespericola columbianus
7a Mature shell width < 19 mm. Periostracum hirsute in some. (Absence of the parietal denticle is occasional in this species, even in otherwise adult shells.) —Cryptomastix mullani, in part
7b Mature shell width 19–35 mm. Periostracum never hirsute — 8
8a Mature shell width 19–24 mm. Malleated sculpture less pronounced; colabral riblets more even and better formed. A weak parietal denticle occasionally present. Southern Interior of British Columbia — Allogona ptychophora
8b Mature shell width 23–35 mm. Malleated sculpture generally stronger; colabral riblets less well-formed. Never with a parietal denticle. Lower Fraser Valley and southern Vancouver Island — Allogona townsendiana