1. Pereopod 7; merus length twice carpus................................................. M. gracilipes Pirlot, 1933
- Pereopod 7; merus similar in length to carpus (or only marginally longer), or carpus distinctly longer than merus.......... 2
2. Pereopod 7; merus as long as, or marginally longer than, carpus; propodus slightly inflated, distinctly broader than carpus in male, length 0.7x carpus. Uropod 1; outer margin of peduncle with relatively long setae. Restricted (?) to Antarctic waters................................................................................... M. setosa (Barnard, 1930)
- Pereopod 7; merus length 0.6–0.7x carpus; propodus slender, length 0.5x (female), 0.6x (male) carpus. Uropod 1; outer ramus with short or minute setae. Not found in Antarctic waters (?)................................. M. galbraithae sp. nov.