“Genus Photinastoma n.g.
Type : Trochus taeniatus Wood
The Falkland Islands taeniatus is very similar in appearance to the genotype of Photinula, but study of the radula and epipodial characters shows that although they are both derivatives from Calliostomid stock they are sufficiently distinct from each other to warrant generic separation. That taeniatus is not a typical Photinula has been pointed out already by Dall (1889a, p. 344), who wrote : 'This species is referred to Photinula by H. & A. Adams, but appears to be simply a smooth Calliostoma. There is no umbilical callus as in Photinula coerulescens ...’, etc., and by Strebel (1908, p. 71), who segregated it from Photinula s.str. under ‘Gruppe Calliostoma-Photinula Strebel'. This hyphenated combination has been cited as of nomenclatural status by Neave (1939, p. 535), but it seems very clearly indicated that Strebel did not intend this. The manner of presentation of subgenera in this same work of Strebel's proves this, i.e. p. 74 ‘Gruppe Promargarita n.subg.' The question remains, that notwithstanding Strebel's intention, does the entry fulfil the then existing requirements for a new name? The mere hyphenating, without alteration, of two valid generic names, is apparently not covered by the rules, and in my opinion does not amount to the proposition of a new genus or subgenus.
Study of the dentition and epipodial fringe in taeniatus and coerulescens confirms that although they compare in a general way with Calliostoma there are sufficient differences between them to warrant separation.
In Photinastoma taeniata the radula is almost identical with that of the Magellan Calliostomids. Also it has four epipodial tentacles as in certain Californian and New Zealand Calliostomids; but not as in the English genotype, which has three. Features of the taeniata radula, which are common to the Magellan calliostomids, and to the species Calliostoma annulata from California but not to the genotype of Calliostoma, are the very deep, narrow-based central tooth and the long 'goose-necked' cusps on the laterals.
In Photinula coerulescens the laterals are not so conspicuously 'goose-necked' and the central is broad and shallow-based, more nearly approaching the typical Calliostomid radula. The epipodial tentacles, however, are six, and there is a distinct but small veil covering the top of each cephalic tentacle at its base.”
(Powell, 1951: 94-95)
Unlike the other genera in the family Calliostomatidae, the genus Photinastoma has almost smooth shells, lacking the beaded sculpture. The protoconch is large. There are some strong spiral cords in the early teleoconch.
Species within the genus Photinastoma include: