Strobilopsidae - Shell Description

Shell Description

The shell is trochiform, dome-shaped or discoidal and umbilicate. The shell has from 4½ to 6 slowly enlarging whorls. The aperture is small, oblique, with armature of 2 or 3 parietal lamellae and several deeply placed basal folds, all growing continuously from an early neanic stage. The peristome is more or less thickened and expanded, the ends of the lip remote, joined by a parietal callus.

The shell, aside from its helicoid shape (not a character of great importance), differs from all Pupillidae in the arrangement of the lamellae and baso-palatal folds. In multidentate Pupillidae the five primary teeth are always recognizable while in Strobilops only the main parietal lamella and the columellar lamella can certainly be said to correspond, and these are found in so many other land shells that their occurrence is not especially significant. It is possible, however, that upper and lower palatal folds of Pupillidae are represented by teeth 5 (the most right basal tooth) and 2 (second left basal tooth), and the basal fold by tooth 1 (the most left basal tooth).

By the accelerated lamellae and folds of the shell, which appear early in the neanic stage, Strobilops resembles various Tornatellininae (within Achatinellidae). In that family both parietal and palatal folds or laminae are sometimes present in the neanic stage. Various pupillid genera also, such as Orcula (Orculidae) and Lauria (Lauriidae), have apertural armature during the neanic stage. Orcula has spiral parietal and columellar lamellae but no basal or palatal folds. Lauria has basal folds, but they are spaced, transverse barriers, wholly unlike the adult basal or palatal armature of the species, and differing equally from the folds of immature Strobilops, which from their inception appear to develop continuously into those of the adult shell. It appears likely that the acceleration or early appearance of apertural armature in Tornatellininae, Orcula, Lauria and Strobilops has been independent in the four groups, and is not indicative of direct relationship between any of them.

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