Type-species.—Callogaza (Microgaza) rotella Dall, 1881: 51; by monotypy.
Diagnosis.—Shell small, circular, depressed, deeply umbilicate, highly iridescent when fresh. Aperture subquadrate, lips thin, columella straight and simple. Sculpture of umbilical plications extending out onto the base and sometimes pustulations or plications at the suture. Operculum thin, corneous, circular, multispiral.
Remarks.Microgaza was introduced as a subgenus of Callogaza Dall, 1881, for M. rotella Dall, 1881. In 1889 he relegated Callogaza to subgeneric rank under Gaza Watson, 1879, and kept Microgaza as a subgenus, placing it in Gaza. The first usage of Microgaza at the generic level was by Dall (1885) in his list of eastern American molluscs. Cossmann (1918) described a fossil species from the European Miocene as Microgaza, but Woodring (1928) expressed doubt that it was actually in this genus. Wood-ring in the same paper described a new subspecies of rotella, vetula, and erected a new subgenus to accommodate his M. cossmanni. M. cossmanni is fossil and vetula was first reported as a fossil, but the GERDA has obtained 6 specimens of vetula to bring the total number of living forms known to 3 (including 1 subspecies).
The systematic position of Microgaza has long been unsettled. Dall's original placement of Microgaza in Callogaza and then Gaza was followed in turn by Cossmann (1918), who included it under Eumargarita Fischer, 1885 (= Margarites Gray, 1847), Thiele (1929), who synonymized it with Solariella Wood, 1842, and Keen (1960), who placed it as a subgenus of Solariella. Examination of the radula of Microgaza rotella rotella indicates a close relationship with Solariella. The radular ribbon is rather short and broad with relatively few rows of teeth. The rhachidian is broad and rounded posteriorly with a very strong cusp bearing two denticles on each side. The admedians and second laterals are similar to each other, each with an inwardly directed triangular cusp, denticulate only on the outer edge. The third lateral is very strong and broad with a rather weak, inwardly directed cusp. The few marginals (6-8 per half-row) are large and sickle-shaped, overhanging the three laterals.