Key to the species of Ariopsis

1a. Osseous medial groove of neurocranium absent (Fig. 3 a, c)....................................................2

1b. Osseous medial groove of neurocranium present (Figs. 3 b, d, e, f)...............................................4

2a. Mesethmoid median portion wide (Fig. 1 a), medial notch of mesethmoid large and shallow (Fig. 3 a)..................................................................................... A. guatemalensis (Mexico to Costa Rica, EP)

2b. Mesethmoid median portion narrow (Figs. 3 b–e), medial notch of mesethmoid narrow and deep (Figs. 3 b–e).............3

3a. Total number of gill rakers on the first plus the second gill arches 31–36, rarely 37.................................................................................. A. assimilis (Quintana Roo, Mexico to Honduras in the Caribbean)

3b. Total number of gill rakers on the first plus the second gill arches 38–44, rarely 36 or 37... A. canteri (Colombia, Caribbean)

4a. Total number of gill rakers on the first plus the second gill arches 40–42....................... A. gilberti (Mexico, EP)

4b. Total number of gill rakers on the first plus the second gill arches 28–37.......................................... 5

5a. Fleshy medial groove of neurocranium conspicuous and very long, always surpassing the posterior margin of eyes (Fig. 4); pterotic lateral margin smoothly convex (Fig. 3).............................................................6

5b. Fleshy medial groove of neurocranium conspicuous or inconspicuous, but never surpassing posterior margin of eyes (Fig. 4); pterotic lateral margin markedly convex, sometimes angled (Fig. 3).............................................7

6a. 13–15 gill rakers on the first gill arch, rarely 16; fenestra delimited by mesethmoid and lateral ethmoid conspicuous (Fig. 3); external posterior branch of lateral ethmoid columnar and thin (Fig. 3)................................................................................................ A. felis (Massachusetts, US, to Yucatán, Mexican Caribbean)

6b. 15–17 gill rakers on the first gill arch; fenestra delimited by mesethmoid and lateral ethmoid inconspicuous (Fig. 3); external posterior branch of lateral ethmoid depressed and thick (Fig. 3)....... A. jimenezi (Archipiélago de las Perlas, Panama, EP)

7a. Lateral margin of sphenotic notched, narrower medially than anteriorly (Figs. 3 and 19)................................................................................................... A. seemanni (El Salvador to Panama, EP)

7b. Lateral margin of sphenotic straight, as wide medially as anteriorly (Figs. 3 and 21)...... A. simonsi (Colombia to Peru, EP)