Diagnosis and definition.—Small frogs (20 to 35 mm. snout-vent length) having digital expansions or not, with transverse groove across tip of each digit; lumbo-inguinal gland prominently elevated, compact, oval, often patterned; axillary glands absent; plantar supernumerary tubercles numerous, more than eight, usually extending between metatarsal tubercles; tarsus lacking tubercles or folds; toes free; terminal phalanges T-shaped; sternum cartilaginous, lacking bony style; sphenethmoid not truncate anteriorly; nasals in contact medially; maxillary and quadratojugal in articular contact; anterior arm of squamosal not in contact with maxillary; dermal cranial elements not involved in integumentary-cranial co-ossification; prevomers large, usually bearing dentigerous processes; maxillary and premaxillary bones dentate; occipital condyles separated; development direct.
Composition.—Ten species.
Distribution.—The southern edge of the Mexican Plateau from Sinaloa to Veracruuz and onto the Oaxaca highlands and Sierra Madre del Sur.
Etymology.—Greek (tomis + dactylus) meaning knife toe; in reference to either the sharp subarticular tubercles or the unwebbed toes.
DISCUSSION
The preceding definitions only slightly alter the present generic limits of Mexican leptodactylids. Two species, previously regarded as Eleutherodactylus, are transferred to the new genus Hylactophryne. The arrangement of the species of Syrrhophus and Tomodactylus remains the same as concluded by Dixon (1957), Duellman (1958), and Firschein (1954) in their reviews of the genera.
Lumbo-inguinal glands are most prominent in the genera Pleurodema and Tomodactylus. Various nondescript glands are present in many genera, but none is so well developed as those of Pleurodema and Tomodactylus.
At least nine leptodactylid genera are either known or thought to be terrestrial breeders lacking a free-living tadpole stage (Eleutherodactylus, Euparkerella, Hylactophryne, Niceforonia, Noblella, Sminthillus, Syrrhophus, Tomodactylus and Trachyphrynus). Niceforonia and Trachyphrynus, and probably Hylactophryne, are not closely related to the other genera. Direct development probably is an adaptation to adverse environmental conditions since many of the species occur in semi-arid or cold (Andean páramos) areas. Eleutherodactylus is generally thought to be the stock from which Euparkerella, Noblella, and Sminthillus evolved (Griffiths, 1959) and from which Syrrhophus and Tomodactylus are derived (Firschein, 1954).
The present distribution of Hylactophryne (isolated on the Mexican Plateau) and its digital form (like that of Papuan and many primitive South American leptodactylids) suggest that the genus was isolated in México throughout the Tertiary, whereas the other Central American genera are either post-Pliocene derivatives of Eleutherodactylus or invaders of Central America from South America since the mid-Pliocene land bridge was formed (Lloyd, 1963).