| Fig. 3. | Diagrammatic views of ventral color patterns in Osteocephalus: |
| a. O. pearsoni, UMMZ 57533, ♂; b. O. taurinus, USNM 166037, ♂; c. O. verrucigerus, KU 123185, ♀. |
Skin.—The dorsal skin of all males of Osteocephalus is tuberculate to varying degrees, whereas the dorsal skin of females is smooth, or nearly so ([Fig. 4]). Osteocephalus verrucigerus differs from other members of the genus by the presence of numerous, large tubercles bearing keratinized tips. The tubercles of leprieurii are numerous and spinous but much smaller than those of verrucigerus; those of taurinus are spinous but less numerous than in leprieurii. Osteocephalus buckleyi has a mixture of large and small, non-spinous tubercles, and pearsoni has only a few, small, scattered, non-spinous tubercles. Fleshy tubercles occur on the eyelids and supratympanic fold in females of buckleyi; a few small tubercles are present on the back of females of pearsoni, whereas the dorsal skin in females of the other species is smooth. The skin on the flanks of both sexes of buckleyi is weakly areolate; in the other species the flanks are smooth. The skin on the top of the head in taurinus is rugose as a consequence of co-ossification. In all species the anal opening is directed posteriorly at the upper level of the thighs.
Hands and Feet.—The feet of Osteocephalus are fully webbed or nearly so. Webbing between fingers one and two is basal in all species. Webbing between fingers two, three, and four is most extensive in taurinus, in which the three fingers are about one-half webbed ([Fig. 5]). Osteocephalus buckleyi, pearsoni, and verrucigerus have reduced webbing between fingers two and three, and leprieurii has reduced webbing between fingers two, three, and four. All members of the genus have well-developed subconical subarticular tubercles on the fingers and toes; there is a tendency for the distal tubercle on the fourth finger to be weakly bifid. Palmar and plantar supernumerary tubercles are well developed in taurinus, moderately developed in buckleyi, leprieurii, and pearsoni, and barely evident in verrucigerus. All of the species have a noticeable fold on the wrist and enlarged prepollices, bearing horny nuptial excrescences in breeding males. The prepollex is least enlarged in buckleyi. Outer metatarsal tubercles are absent. The inner metatarsal tubercle is moderately well developed and ovoid in leprieurii and pearsoni; it is elliptical and flat in the other species. Tarsal folds are absent in all species except verrucigerus, in which the folds are barely evident.
| Fig. 4. | Segments of dorsal skin of males of Osteocephalus showing size and arrangement of tubercles: |
| a. O. verrucigerus, KU 123183; b. O. taurinus, USNM 166033;
c. O. leprieurii, KU 126616; d. O. buckleyi, USNM 165999. Each square = 1 sq. cm. |
Cranium.—As a genus, the cranial structure is remarkably uniform and quite generalized when viewed in the context of the family Hylidae. The skulls are broad and relatively flat, each being only slightly more broad than long and about one-third as high as long. In dorsal aspect the snouts are broadly rounded; the snout of buckleyi is somewhat less rounded and appears to be slightly longer than the snouts of other species. This subtle difference relates to the relative narrowness of the premaxillaries in buckleyi.
