1 Distal enlargement of the carpus of the second pereiopod gradual (Figure 6 a); rostrum distinctly surpasses the scaphocerite; rostrum has an evident portion without teeth (Figure 7 a); internal pair of posterior spine of the telson at most 2 times longer than the median apex in small individuals; internal pair of posterior spine of the telson does not reach the median apex in adults (Figure 8 a).............................................................................. M. amazonicum

- Rostrum reach or slightly overreach end of the scaphocerite; rostrum does not have an evident portion without teeth (Figures 7 b and 7c); internal pair of posterior spine of the telson surpasses the median apex and is more than 2 times longer (Figures 8 b and 8c)............................................................................................. 2

2 Carpus of the second pereiopod similar to the chela length, chela longer than half of the carapace length, distal enlargement of the carpus gradual (Figure 6 b)............................................................... M. acanthurus

- Carpus of the second pereiopod noticeably longer than the chela, distal enlargement of the carpus abrupt (Figure 6 c)..................................................................................................... M. jelskii