Distribution.—Hyla robertsorum inhabits streams in the pine and fir forests in the higher parts (2250 to 3050 meters) of the Sierra Madre Oriental in extreme northern Puebla and eastern Hidalgo (Fig. 4).
Specimens examined.—Hidalgo: 16 km. W Agua Blanca, UMMZ 106432 (6); El Chico Parque Nacional, CNHM 75786, 100124, KU 57650-71, 59824-5 (skeletons), 59914-5 (skeletons), 71269-95, 71757 (skeleton), UIMNH 10349-64, 27022-35, 39434-49, UMMZ 92462, 106401 (5), 106443 (tadpoles), USNM 114762-85, 134268; 3.3 km. N Zacualtipán, KU 53810, 60078 (tadpoles); 8.5 km. SE Zacualtipán, KU 60079 (tadpoles). Puebla: Honey, UMMZ 95245.
Hyla pachyderma Taylor, Univ. Kansas Sci. Bull., 28:308-310, pl. 27, figs. 1-4, November 12, 1942 [Holotype.—USNM 115029 from Pan de Olla, Veracruz, south of Tezuitlán, Puebla, México; Hobart M. Smith collector]. Taylor and Smith, Proc. U. S. Natl. Mus., 95:588, June 30, 1945. Smith and Taylor, Bull. U. S. Natl. Mus., 194:86, June 17, 1948; Univ. Kansas Sci. Bull, 33:350, March 20, 1950. Rabb and Mosimann, Occ. Papers Mus. Zool. Univ. Michigan, 563:7-8, March 29, 1955.
Diagnosis.—Maximum snout-vent length in males 40 mm.; snout in dorsal profile round; tarsal fold strong; inner metatarsal tubercle round and moderate in size; outer metatarsal tubercle small and indistinct; webbing on foot extending to middle of penultimate phalanx of fourth toe; nuptial spines on thumb large; thoracic fold present; anal opening at level of middle of thighs; dorsum dull grayish brown with scattered indistinct dark flecks; venter cream-color mottled with brown on throat and chest; flanks grayish brown with cream-colored reticulations; anal stripe distinct, creamy white, sometimes extending outward on thighs; white spots or line below anus; vocal slits absent.
Description.—The following description is based on USNM 115028 from Pan de Olla, Veracruz. Adult male having a snout-vent length of 39.9 mm.; tibia length, 21.0 mm., 52.6 per cent of snout-vent length; foot length (measured from proximal edge of inner metatarsal tubercle to tip of longest toe), 20.5 mm.; greatest width of head, 12.8 mm., 32.1 per cent of snout-vent length; head length, 12.3 mm., 30.8 per cent of snout-vent length. Snout short, in lateral profile bluntly rounded, in dorsal profile rounded; canthus rounded; loreal region slightly concave; lips thick, round, and not flaring; nostrils slightly protuberant; internarial distance, 2.7 mm.; interorbital distance, 3.7 mm., somewhat broader than eyelid, 2.9 mm. A heavy dermal fold from posterior corner of eye above tympanic region and then to insertion of forearm; tympanum completely concealed. Forearm moderately robust; distinct fold on wrist; prepollex enlarged bearing cluster of moderate-sized, horny, nuptial spines continuous on edge of digit; row of spines present on second finger; subarticular tubercles round, small proximally and slightly larger distally; supernumerary tubercles small and indistinct; three palmar tubercles, median and outer partly fused; fingers long, moderately slender; discs moderately large; length of fingers from shortest to longest, 1-2-4-3; second and fourth fingers subequal in length; webbing between fingers rudimentary. Heels overlap by about one-fourth length of shank when hind limbs adpressed; tibiotarsal articulation extends to anterior edge of eye; tarsal fold thick, low, extending nearly to heel; inner metatarsal tubercle moderately large and round; outer metatarsal tubercle small and indistinct; subarticular tubercles small and round; supernumerary tubercles small, present on proximal segments of digits; toes moderately long and slender; length of toes from shortest to longest, 1-2-3-5-4; third and fifth toes subequal in length; toes about three-fourths webbed; web extending to middle of penultimate phalanx of fourth toe; discs rather small, about two-thirds size of those on fingers. Anal opening directed posteroventrally at level of middle of thighs; anal flap slightly elongate; thick, transverse dermal fold above anus. Skin of dorsum and ventral surfaces of limbs, except thighs, smooth; skin of chin, belly, and ventral surfaces of thighs granular; thoracic fold present. Tongue nearly round, slightly notched behind, and barely free posteriorly; vomerine teeth 3-3, situated on posteroventral edges of small, transverse vomerine ridges between rather large triangular inner nares; no vocal slits.
Color (in alcohol) of dorsal surfaces of head, body, and limbs dull grayish brown with indistinct scattered darker flecks; flanks grayish brown with cream-colored reticulations; posterior surfaces of thighs tan; chin cream-color, mottled with brown; belly creamy yellow; anal stripe cream-color.
Variation.—In addition to the specimen described above three others are known. One is a juvenile having a snout-vent length of 29.5 mm., and two are females having snout-vent lengths of 46.9 and 41.6 mm. Variation in structure and coloration between the four specimens is slight. In the females the tympani are partly visible and are about one-third the diameter of the eye; the chest is mottled with brown; the anal stripe extends laterally in the form of a row of cream-colored dashes and spots onto the posterodorsal surfaces of the thighs.
Remarks.—On the basis of the four specimens available for study, Hyla pachyderma seems to be closely related to Hyla crassa and Hyla robertsorum. In the Hyla bistincta group, Hyla pachyderma is unique in having enlarged nuptial spines.
Taylor and Smith (1945:588) stated that the frogs were found on bushes and weeds beside a small, bounding stream near Pan de Olla. I have searched unsuccessfully for this species in the area around Pan de Olla and Tezuitlán.