Lateral views of heads of (top) Plestiodon copei (ROM 19857) and (bottom) Mesoscincus schwartzei (ROM 10377), both formerly within the genus Eumeces. The arrows point to presubocular scales (Mesoscincus species have three, a derived character state; Plestiodon copei shown for comparison). L refers to labial scales.
About Mesoscincus: There are three poorly known skinks, formerly genus Eumeces, in Central America, now placed in Mesoscincus. These are the Tepalcatepec Skink, Mesoscincus altamirani (Dugés, 1891), known from Mexico, Managua Skink, Mesoscincus managuae (Dunn, 1933), from Costa Rica, Nicaragua, Honduras and El Salvador, and Schwartz' Skink, Mesoscincus schwartzei (Fischer, 1884), from Mexico, Guatemala and Belize.
The genus diagnosis (by Griffith et al., 2000): 27 presacral vertebrae. Limbs relatively slender, lamellae not expanded. Dorsal surface of head somewhat depressed in lateral view, parietal bone with clear lateral indentations and supratemporal fontanelle open. Sexual dimorphism not distinct. Scales shiny, separated by shallow sutures. Two loreals, followed by three presubocular scales. Post-nasal scales present. Palpebral and superciliary scales not separated by groove. Elevated numbers (4 or 5 pairs) of nuchal scales, followed by several pairs of broadened mid-dorsal scales and broad row of fused mid-dorsal scales. Large medial preanal scales overlie small lateral pair. Ear lobules small and rounded, but conspicuous. Color pattern variable, but generally consists of irregular spots and longitudinal striping. From lowlands of southern Mexico (Michoacan, Yucatan Peninsula), northern and Pacific coastal Honduras, Pacific coastal Nicaragua and Costa Rica.
Griffith, H., A. Ngo and R. W. Murphy. 2000. A cladistic evaluation of the cosmopolitan genus Eumeces Wiegmann (Reptilia, Squamata, Scincidae). Russ. J. Herpetol. 7 (1): 1-16.
And for those with a molecular bent:
Schmitz, A., P. Mausfeld and D. Embert. 2004. Molecular studies on the genus Eumeces Wiegmann, 1834: phylogenetic relationships and taxonomic implications. Hamadryad 28 (1-2): 73 – 89.
But always remember, it's not the molecules that matter. It's the drawings.