| Fig. 7. | Dorsal views of vertebral columns of two species of Osteocephalus: |
| a. O. leprieurii, KU 125962, ♀; b. O. buckleyi, USNM 165997, ♀. ×2. |
Vertebral Column.—The cervical cotyles are uniformly widely displaced. The neural arches are low and non-imbricate. The transverse processes of the third presacral vertebrae are approximately equal in width to the sacral diapophyses in all species, except buckleyi, in which the processes of the third presacral are slightly narrower than the diapophyses. Osteocephalus buckleyi is further distinguished by the presence of narrow transverse processes on presacrals five through eight ([Fig. 7b]); males have narrower processes than do females. The processes are moderately wide but subequal in width in pearsoni, taurinus, and verrucigerus, whereas they are nearly equivalent in width to one another and to the sacral diapophyses in leprieurii ([Fig. 7a]). The sacral diapophyses of all species are moderately dilated and posterolaterally inclined. The coccyx bears a distinct dorsal ridge and has a bicondylar articulation with the sacrum.
Pectoral Girdle.—The pectoral girdles are fully arciferal and bear small, cartilaginous omosterna and moderately large cartilaginous sterna. The coracoids are robust, and the clavicles are strongly arched. Procoracoid cartilage seems to be absent. The scapulae are large, longer than the clavicles, and bicapitate proximally. The suprascapulae are moderately large and well ossified in leprieurii and taurinus. The suprascapula of verrucigerus is poorly ossified, and that of buckleyi is not ossified.
Pelvic Girdle.—The ilia of buckleyi, taurinus, and verrucigerus lack any indication of a crest on the shaft, whereas leprieurii has a low crest. The dorsal acetabular expansion of the ilia is moderately low in taurinus and verrucigerus, but distinctly lower in buckleyi and leprieurii. The ilia of all species bear low dorsal protuberances. The ischia of leprieurii, taurinus, and verrucigerus are moderately expanded; that of buckleyi is somewhat less expanded dorsally. The pubis of leprieurii, taurinus, and verrucigerus are calcified, whereas that of buckleyi remains cartilaginous.
Throat Musculature and Vocal Sac Structure.—Tyler (1971) described the throat myology of Osteocephalus. The genus is characterized by a moderate-sized araphic submentalis muscle and an undifferentiated intermandibularis having an elongate median aponeurosis. The intermandibularis and submentalis are completely independent in buckleyi, whereas in the other species there is a small attachment between these muscles.
According to Tyler (pers. com.), Osteocephalus has three distinctive types of vocal sac structure which result from differences in the development of the interhyoideus muscle and the overlying skin. In leprieurii and verrucigerus the supramandibular portions of the interhyoideus form a simple tubular, posterolateral extension; there is no modification of the associated skin. Osteocephalus buckleyi and pearsoni have more extensive development of the supramandibular portions of the interhyoideus; furthermore, the associated skin forms a broad, loose fold extending from the ventromedial surface of the throat dorsolaterally to the base of the supratympanic fold. Like buckleyi and pearsoni, the supramandibular portion of the interhyoideus is much expanded in taurinus. The vocal sac structure of taurinus differs from that of other members of the genus in that the skin of taurinus forms an everted pouch, which dangles loosely beneath the supratympanic fold.
Key to the Species of Osteocephalus
| 1. | Inner edge of third finger webbed to mid-length of antepenultimate phalange; dorsum brown with dark brown spots or median blotch; skull in adults casqued and co-ossified with prominent supraorbital flanges |
| [O. taurinus] | |
| Inner edge of third finger webbed to base of antepenultimate phalange; dorsum plain or marked with dark blotches or transverse bars; skull in adults smooth or slightly exostosed, lacking supraorbital flanges | |
| 2 | |
| 2. | Skin on flanks areolate; dorsum in males bearing a mixture of large and small non-spinous tubercles; lips distinctly barred |
| [O. buckleyi] | |
| Skin on flanks smooth; dorsum in males bearing tubercles of uniform size; lips not barred | |
| 3 | |
| 3. | Dorsal pattern consisting of narrow transverse dark bars; dorsum in males bearing numerous small spinous tubercles |
| [O. leprieurii] | |
| Dorsal pattern not consisting of transverse bars; dorsal tubercles large or few in number | |
| 4 | |
| 4. | Dorsum uniformly dark brown; venter heavily mottled with black, especially in females; dorsum in males bearing large, keratinized tubercles |
| [O. verrucigerus] | |
| Dorsum tan with irregular dark brown blotches; venter cream with fine brown reticulations; dorsum in males bearing few, small non-spinous tubercles | |
| [O. pearsoni] | |