1. Pereopods 5–7 simple, dactyl not retractile................................................................. 2
- Pereopod 5 weakly subchelate with dactyl closing against propodus. Pereopod 7 with partly retractile dactyl............. 6
2. Pereopod 5; propodus not narrowing distally with tuft of 3–4 setae near anterodistal corner........................... 3
- Pereopod 5; propodus narrows evenly distally, without tuft of setae as above...................................... 4
3. Pereopods 4 & 5 similar in length; with P 6 marginally longer.............................. M. carlsbergi Zeidler, 2009
- Pereopods 5 & 6 (especially P 5) distinctly shorter than P 4.................................... M. antarctica sp. nov.
4. Pereopod 5 distinctly longer than P 4. Telson length about 0.7 x, or more, peduncle of U 3........ M. beebei Shoemaker, 1945
- Pereopod 5 distinctly shorter than P 4. Telson length about 0.5 x, or less, peduncle of U 3............................. 5
5. Pereopods 3–7; dactyl very short, about 0.2 x, or less, length propodus................... M. diomedeae Woltereck, 1909
- Pereopods 3–7; dactyl relatively long, about 0.4–0.5 x length propodus..................... M. macronyx Barnard, 1932
6. Pereopod 5; propodus with long, strong setae along posterior margin, forming weak subchela with dactyl; dactyl length about half propodus. Pereopod 7 with relatively long, almost straight, partly retractile dactyl...... M. subchelata Vinogradov, 1964
- Pereopod 5; propodus with few short setae posteriorly, forming barely perceptible subchela with dactyl; dactyl length much less than half propodus. Pereopod 7 with short, curved, partly retractile dactyl, forming weak subchela with propodus...................................................................................... M. mixta Vinogradov, 1964