1A. Rostral sensory system 4+2, without four large pits at margin of frontal bones; scales around body 20–27, modally 22................................................................................................. M. gilberti
1B. Rostral sensory system 4+2+4, with four large deep pits at margin of frontal bones (in largest specimens the two intermediate pits may fuse with the two posterior pits); scales around body 13–19, modally 16................................... 2
2A. Anus located just anterior to origin of anal fin; tips of pelvic fins not reaching anterior margin of vent; predorsal scales 21–26, modally 23–24; posterior margins of scales laciniate................................................ M. martinica
2B. Anus positioned closer to base of pelvic fin than to origin of anal fin; tips of pelvic fins reaching or extending beyond anterior margin of vent; predorsal scales 16–21, modally 19–20; posterior margins of scales smooth or laciniate................ 3
3A Posterior margins of predorsal and lateral scales strongly laciniate............................................... 4
3B Posterior margins of predorsal and lateral scales smooth or irregular............................................. 5
4A Lateral scales 38–39; predorsal scales 16–18, modally 18; length of spinous dorsal fin 6.4–6.9% SL, mean = 6.8%; least depth of caudal peduncle greater than 9.2–9.5% SL, mean = 9.4%........................................... M. argentea
4B. Lateral scales more than 40; predorsal scales 20–21, modally 21; length of spinous dorsal fin 8.8–9.0% SL, mean = 8.8%; least depth of caudal peduncle 7.4–8.6% SL, mean = 8.2%.......................................... M. procera sp. nov.
5A. Precaudal vertebrae 17–18; pre-anal-fin length 6.3–6.4% SL, mean = 63.6%; orbit diameter 6.5–6.7% SL, mean = 6.6%; lateral stripe becomes diffuse at base of caudal fin, lacking a distinctive mark.................................. M. dissimilis
5B. Precaudal vertebrae 14–16, modally 15; pre-anal-fin length 5.6–6.2% SL, mean = 6.0%; orbit diameter 6.8–8.5% SL, mean = 7.7%; lateral stripe ends distinctly before hypural base there can be a diffuse spot or crescent mark present...............6
6A. Least depth of caudal peduncle 8.5–10.5% SL, mean = 9.7%; body depth 15.7–18.1% SL, mean = 16.9%; total gill rakers, first arch 15–18, modally 16–17....................................................................... M. analis
6B. Least depth of caudal peduncle 6.7–8.5% SL, mean = 7.7%; body depth 13.5–15.9% SL, mean = 14.8%; total gill rakers, first arch 16–20, modally 18 or 19............................................................ M. pygmaea sp. nov.