1. Gnathopod 1 carpus with posterodistal margin produced into 2 or more spines..................................... 2
- Gnathopod 1 carpus with posterodistal margin produced into a single spine........................................ 8
2. Gnathopod 1 carpus with outer of two spines the longer....................................................... 3
- Gnathopod 1 carpus not as above......................................................................... 5
3. Gnathopod 1 carpus with very long setae on the distal margin.............................. M. sporadhi Myers, 1969b
- Gnathopod 1 carpus lacking very long setae on the distal margin................................................ 4
4. Gnathopod 1 carpus with primary spine slender, secondary spine not arising from margin of primary spine........................................................................................... M. chelifer (Spence Bate, 1862)
- Gnathopod 1 carpus with primary spine broad based, secondary spine arising from anterior margin of primary spine............................................................................................... M. similis Myers
5. Gnathopod 1 carpus with central of three spines the longest............................. M. stationis Della Valle, 1893
- Gnathopod 1 carpus with inner of two or more spines the longest................................................ 6
6. Gnathopod 2 basis with anterior margin strongly convex and crenulate...................... M. gryllotalpa Costa, 1853
- Gnathopod 2 basis not as above.......................................................................... 7
7. Gnathopod 1 carpus with outer of two spines arising from posterior margin...................... M. bifidus Myers, 1977
- Gnathopod 1 carpus with outer of two spines arising lateral to the posterior margin............ M. anomalus (Rathke, 1843)
8. Gnathopod 2 propodus with spine at posterior distal corner..................................................... 9
- Gnathopod 2 propodus without spine at posterior distal corner................................................. 10
9. Gnathopod 2 carpus and propodus subequal in length and breadth......................... M. armatus Chevreux, 1887
- Gnathopod 2 carpus longer and more slender than propodus........................... M. chelifer (Spence Bate, 1862)
10. Gnathopod 2 very elongate and slender with long pectinate setae on anterior margin of carpus and propodus...................................................................................... M. versiculatus (Spence Bate, 1856)
- Gnathopod 2 not as above.............................................................................. 11
11. Gnathopod 1 coxa irregularly rounded, not produced anteriorly............................. M. obtusatus Myers, 1973
- Gnathopod 1 coxa strongly produced anteriorly, acute or subacute.............................................. 12
12. Gnathopod 1 carpus with posterodistal margin produced into a slender spine, dactylus slender... M. anomalus (Rathke, 1843)
- Gnathopod 1 carpus with posterodistal margin produced into a stout triangular spine, dactylus stout................... 13
13. Gnathopod 1 coxa anteriorly produced into a slender spine. Gnathopod 2 basis weakly expanded, carpus and propodus short and stout..................................................................... M. algicola Della Valle, 1893
- Gnathopod 1 coxa only moderately produced anteriorly, subacute. Gnathopod 1 basis strongly expanded, carpus and propodus slender, elongate...................................................................... M. periergos sp. nov.