Piatt (1934) presented arguments against assigning Eleutherodactylus latrans to the genus Lithodytes and concluded that it was a "true" Eleutherodactylus. Contrary to his arguments, latrans (= augusti of Zweifel) and E. tarahumarensis Taylor differ from all other Eleutherodactylus (and Syrrhophus and Tomodactylus) in the nature of the tips of the digits (external and skeletal). The digits of Hylactophryne are like those of Eupsophus. My study of nearly all genera of leptodactylids indicates that Noble (1925) was correct in suggesting that Borborocoetes (= Eupsophus) is a close relative of Eleutherodactylus latrans, although Noble's arguments were based in part upon false evidence concerning the breeding habits of E. latrans, then thought to have a free-living tadpole.
Kellogg (1932) and Piatt (1934) argued that the terminal phalanges of E. latrans were typically eleutherodactyline. The variation of this character in Eupsophus (see [Fig. 4]) ranges from knobbed to bifurcate or Y-shaped (T-shaped in Eleutherodactylus, Syrrhophus and Tomodactylus) and encompasses the nature of the character represented in Hylactophryne. Eupsophus differs from Hylactophryne in possessing a frontoparietal fontanelle, in generally having a maxillary-quadratojugal gap, and in having a free swimming tadpole stage.
Fig. 5. Outline drawings of Leptodactylus melanonotus (left, KU 65704, × 2) and Eleutherodactylus alfredi (right, KU 93994, × 2).
KEY TO MEXICAN LEPTODACTYLID GENERA
| 1. | Small (20-40 mm.), pustular, toadlike frogs; maxillary and premaxillary bones not bearing teeth | Engystomops |
| Large (20-110 mm.), smooth skinned and non-toadlike frogs; maxillary and premaxillary bones bearing teeth | 2 | |
| 2. | No conspicuous waist ([Fig. 5]); sternum bearing bony style, | Leptodactylus |
| Constrictions at waist ([Fig. 5]); sternum cartilaginous, no bony style | 3 | |
| 3. | Few (less than six), if any, supernumerary tubercles on plantar surface | Eleutherodactylus |
| Many (more than 8) supernumerary tubercles on plantar surfaces | 4 | |
| 4. | Terminal, transverse groove across tip of digits, especially outer two fingers, digits expanded or not; small frogs (18 to 40 mm.) | 5 |
| Tips of digits lacking transverse groove; digits unexpanded; medium-sized to large frogs (37 to 94 mm.) | Hylactophryne | |
| 5. | Lumbo-inguinal gland compact, oval | Tomodactylus |
| Lumbo-inguinal or inguinal gland absent or diffuse and irregular in outline | Syrrhophus |