both left- and right-handed coiling are found in the same population. Sinistral mutants of normally dextral species and dextral mutants of normally sinistral species are rare but well documented occurrences among land snails in general. Populations or species with normally mixed coiling are much rarer, and, so far as is known, are confined, with one exception, to a few genera of arboreal tropical snails. Besides Amphidromus, the Cuban Liguus vittatus (Swainson), Haitian Liguus virgineus (Linnaeus) (family Orthalicidae), some Hawaiian Partulina and many Hawaiian Achatinella (family Achatinellidae), as well as several species of Pacific islands Partula (family Partulidae), are known to have mixed dextral-sinistral populations. A possible exception may concern some of the European clausiliids of the subfamily Alopiinae. They are obligatory calciphiles living in isolated colonies on limestone outcrops. Several sets of species differ only in the direction of coiling, but the evidence is inconclusive as to whether left- and right-handed shells live together. Soos (1928, pp. 372–385) summarized previous discussions of the problem and concluded that the right- and left-handed populations were distinct species. Others have stated that these populations were not distinct, and the question is far from settled. The Peruvian clausiliid, Nenia callistoglypta Pilsbry (1949, pp. 216–217), also has been described as being an amphidromine species. The genetics of reverse coiling in a rare dextral mutant of another clausiliid, Alinda biplicata (Montagu), has been studied by Degner (1952). The mechanism is the same as in Radix peregra (Müller), with the direction of coiling determined by a simple Mendelian recessive. Standard ways of viewing a shell In photographs or illustrations, a gastropod shell can be shown oriented in a number of standard ways: apertural view: this is the most common viewing angle. The shell is shown in its entirety, with its aperture facing the viewer, and the apex at the top. If the aperture is on the right side when viewed like this, then the shell-coiling is "right-handed" or dextral; if the aperture is on the left side when viewed like this, the shell has "left-handed" or sinistral shell-coiling. abapertural view (or dorsal view): the shell is shown with its aperture 180° away from the viewer, and with the apex at the top. umbilical view (or basal view): the shell is shown viewed directly from below. In most cases where there is an umbilicus, this is in clear view. apical view: the shell is shown looking down directly