1A.	Flowers 3-merous	M. guaranitica
1B.	Flowers 4–6-merous	2
2A.	Flowers almost always 4-merous; stamens always 4; if 5 stamens, then only 1 pair of pinnae per leaf and 2 pairs of leaflets per pinna, but the inner leaflet of the proximal pair reduced or absent	3
2B.	Flowers 4–6-merous; stamens 5–10	5
3A.	Leaflets 11–36 pairs per pinna; flowers always 4-merous; stamens 4	M. pudica ssp. hispida
3B.	Leaflets 2 pairs per pinna; flowers 4–5-merous; stamens 4–5	4
4A.	Pod thinly strigulose and pulverulent or glabrate	M. albida ssp. glabrior
4B.	Pod, including valves and replum coarsely strigose	M. albida ssp. albida
5A.	Stems, leaf rachises and frequently leaflets margins with bulbous or dilatated basally, flagelliform setae	6
5B.	Stems leaf rachises and frequently leaflets margins lacking with bulbous or dilated basally, flagelliform setae	7
6A	Herbaceous, the stems compressed, rooting at nodes; capitula elliptic or short cylindric	M. strigillosa
6B	Stems woody, erect, never compressed, never rooting at nodes; capitula subglobose	M. pigra ssp. asperata
7A	Stems and rachis of leaves serially armed with curved thorns	8
7B.	Stems unarmed, or if armed, then with groups of 1–3 thorns per node or internode, if the thorns are more numerous, then straight or nearly so and the rachis of the leaves rarely thorny	12
8A.	Flowers always 4-merous; scandent sub-shrubs; filaments white (capitula with white appearance)	M. malacophylla
8B.	Flowers 5-merous; herbaceous prostrate; filaments pink (capitula with pink appearance)	9
9A.	Leaflets with evident reticulate venation	M. paucijuga
9B.	Leaflets with no evident reticulate venation	10
10A.	Larger leaves with the petiole 2.5–3.5 times longer than the rachis leaf (Tamaulipas)	M. latidens
10B.	Larger leaves with the petiole 1.5–2.5 times longer than rachis leaf	11
11A.	Pinnae 1–3 pairs per leaf; leaflets with rounded or obtuse apices; fruit 3–5 cm long	M. potosina
11B.	Pinnae 3–4 pairs per leaf; leaflets with acute apices; fruit 6–8 cm long	M. monclovensis
12A.	Plants unarmed; leaflets 1–4 cm long	13
12B.	Plants armed with thorns; leaflets shorter than 1 cm long	14
13A.	Each pinna with 2–5 pairs of leaflets; fruit 6–8.5 mm wide	M. leucaenoides
13B.	Each pinnae with 1pair of leaflets; fruit 9–12 mm wide	M. martin delcampoi
14A.	Leaves unipinnate; leaflets 1–3 pairs per pinna arranged on a primary axis	M. unipinnata
14B.	Leaves bipinnate; leaflets developing into secondary axes	15
15A.	Largest leaves with 5 or more pairs of pinnae or largest pinnae with 9 or more pairs of leaflets	M. biuncifera
15B.	Largest leaves with 1–4 pairs of pinnae or largest pinnae with 10 or fewer pairs of leaflets	16
16A.	Infrapetiolar thorn arising immediately below the node, alone or associated with 1–2 infrastipular thorns, these displaced towards the internode	17
16B.	Infrapetiolar thorn solitary or associated with 1–2 infrastipular thorns, but all 3 displaced towards the internode	18
17A.	Leaflets sericeous in both faces; fruit is a craspedium; all capitula or some of them axillary on new branchlets	M. monancistra
17B.	Leaflets glabrous; fruit with valvate dehiscence; all capitula arising from brachyblasts	M. texana
18A.	Leaflets or puberulent; valves of the pod puberulent, velutinous, hispid, villous or setose	19
18B.	Leaflets and fruits glabrous	21
19A.	Fruits densely setose; the setae in turn setose with the apex slightly uncinulate; leaves 4.5–7 cm long, pinnae 3–5 pairs per leaf	M. setuliseta
19B.	Fruits with villous and uncinulate setae; leaves 2.8 cm long or shorter; pinnae 1–3 pairs per leaf	20
20A.	Calyx 0.8–1 mm long, ciliate; fruit with cylindrical setae, villous at the base and uncinulate apically; leaves with sericeous pubescence	M. emoryana ssp. emoryana
20B.	Calyx 0.5–0.8 mm long, conspicuously ciliated; fruit with flattened villous setae, uncinulate apically; leaves with dense sericeous pubescence	M. emoryana ssp. canescens
21A.	Petioles rounded dorsally with a ventral groove; flowers sessile; calyx 0.9–1.5 mm long; leaves with 1 pair of pinnae, each pinna with 1 pair of leaflets (rarely 2 pairs); valves separating from replum in a single piece	M. zygophylla
21B.	Petioles dorsoventrally compressed, with 2 grooves, both dorsally and ventrally; short-stalked flowers; calyx 0.4–0.9 mm long; leaves with 2 or more pairs of pinnae and pinnae with 2 or more pairs of leaflets	M. turneri