Dulack Richards made an interesting contribution to the Facebook group Tree and Land Snails, by publishing this photo:
According to Richards on the top row: Neopetraeus arboriferus paucistrigatus [Weyrauch, 1967; type locality: Peru, La Libertad, road from Huamachuco to Pataz, Chagual], on the bottom row: Neopetraeus binneyanus [(L. Pfeiffer, 1857); type locality: Peru, prov. Pataz]. His question was: are the two other rows hybrids?
This phenomenon has more often been observed, albeit it is seldom, in other land snail species (see for examples from South America Breure, 2008). While it is the only case I know from Neopetraeus, there are different Bostryx species where transitional series can be found. Research has been done in the río Rimac valley near Lima where a similar case can be found. Based on measuments of the microclimate and molecular studies, provisional results point to ecological factors to explain this transition; it may be regarded as a cline and this does not allow for the delineation of (sub)species. So, at least, morphologically these shells look like hybrids, but it may be possible that a study of the DNA would reveal a more nuanced picture.
Remains the question: were these shells collected at the same locality and, if so, where? It may be noted that the type locality of N. binneyanus was very imprecise, so from a scientific point of view it is interesting to have a clue where this cline originated from. On my question to Richards to disclose the locality he did not provide an answer, but given the type locality of Weyrauch’s taxon my guess is it was in the vicinity of Chagual (see also p. 511 in my 2008 paper).
Breure, A.S.H., 2008. Carination strikes the eye: extreme shell shapes and sibling species in three Andean genera of the Orthalicidae (Gastropoda, Stylommatophora). – Zoologische Mededelingen, 82: 499-514.