Taxonomy of the Struthiolariids

Recent practice as to generic nomenclature in this family has varied. Struthiolaria, Pelicaria and Tylospira are each undoubtedly natural divisions, and as such were used sectionally by Finlay (1927) and Powell (1951). Morton (1950) and Marwick (1951, 1952) have employed them as full generic names. Commendable though it be to keep innovations in nomenclature at a minimum, it is, nevertheless, clear that in the Struthiolariidae the relatively slight morphological differences between Struthiolaria and Pelicaria conceal long-standing differences, separating two large groups of species in time past. Separate names in some form are needed, and deserve to be recognised; it is scarcely reasonable to require our taxonomic system to be simpler than the observed facts it has to express, and in the case of well-charted fossil lineages, these facts are likely to be complex. On the other hand the plight of the general zoologist interested in one or more aspects of the mollusca should not be left out of account; and in a case where three living species alone survive, forming together an undoubtedly close natural group, the student may rightly be dismayed to find them provided with a genus each. These three names, together with Callusaria (Finlay, 1927) and Singletonaria (Marwick, 1952) can probably be best employed sectionally. Struthiolaria may remain the inclusive genus, without detriment to a natural arrangement, and for precision of reference the sectional or group name may be inserted in its brackets whenever the nature of the discussion requires it.

Perissodonta may well remain a full genus. It forms a clear-cut group, with two species strikingly alike in conchological, radular and opercular features. In the aporrhaid facies of the shell, the absence of an incubatory pouch in the female, and in the primitive characters of the radula, Perissodonta forms a valuable intermediate genus. In the specialisation of the gill and in the development of ciliary feeding, as well as in the possession of the clawed operculum, it is equally clearly a good struthiolariid.