umbilical) surface uppermost, and because of this, the shells appear to be dextral. Indeed, formerly planorbids were thought to have dextral shells, and so species of this family were figured as if they had dextral shells. Although it is now understood that these species are sinistral in shell coiling, disk-like Planorbid shells are often still shown in illustrations oriented as if they were dextral. Most species of coiled planorbids have a rather thin and moderately smooth shell, although more distinct sculpture such as a keel occurs in, and is diagnostic of, certain species. In the flat, keeled species, the whorls tend to overlap. The aperture has a sharp outer lip. A peristome can be present, but often the lip is not thickened nor reflected. Those planorbid species which have a high-spired shell may have a narrow umbilicus, but frequently this is covered by callus. In height most species vary between 6 mm and 6 cm, however, disk-like shells are usually less than about 2 cm in maximum dimension. Like all pulmonate aquatic snails, ramshorn shells do not have an operculum to close the shell aperture. Sinistral shells Flat-coiled planorbid gastropod shells are hard to understand in terms of their coiling and orientation. Many of the shells of species in this family are almost planispiral in coiling such that one side of the shell often looks rather like the other side, but it is important to bear in mind that nonetheless there is an umbilical side and a spire side of the shell. In addition these are in fact sinistral shells, despite the fact that the snail carries its shell as if it were a normal dextral shell. To make sense of the shell coiling, the following facts are useful: In life, these pond snails hold their shells upside down compared to the normal gastropod shell orientation, with the umbilicus facing upwards. The spire of the shell is quite sunken in many species. In addition, it is carried facing downwards. The umbilicus of the shell is very wide and shallow. In some species the umbilicus is not as deeply "dished" as the sunken spire is, so superficially it can be hard to tell one from the other. However, once it is understood that the planorbid shell is sinistral, if the shell is held with the aperture on the left and facing the observer, then the sunken spire side of the shell