Description.—The following description is based on UIMNH 25050 from Cerro San Felipe, Oaxaca. Adult male having a snout-vent length of 53.7 mm.; tibia length, 26.9 mm., 50.1 per cent of snout-vent length; foot length (measured from proximal edge of inner metatarsal tubercle to tip of longest toe), 25.4 mm.; greatest width of head, 17.6 mm., 32.8 per cent of snout-vent length; head length, 16.0 mm., 29.8 per cent of snout-vent length; diameter of eye, 5.4 mm.; diameter of tympanum, 1.5 mm., 27.8 per cent of diameter of eye. Snout short, in lateral profile bluntly rounded, in dorsal profile broadly round; canthus absent; loreal region nearly flat; lips thick and not flaring; nostrils barely protuberant; internarial distance, 3.8 mm.; interorbital distance, 4.7 mm., somewhat broader than width of eyelid, 3.8 mm. Heavy dermal fold from posterior corner of eye above tympanum and then to insertion of forearm; tympanum concealed above, its diameter about equal to its distance from eye. Forearm thick; distinct fold on wrist; prepollex enlarged bearing patch of small nuptial spines continuous on side of digit; similar patch on second finger; subarticular tubercles small and round, none bifid; few supernumerary tubercles on proximal segments of digits; large, flat palmar tubercle; fingers long and slender; length of fingers from shortest to longest, 1-2-4-3; discs moderately large; rudimentary web between second and third fingers and between third and fourth. Legs thick; heels overlap by about one-fourth length of shank when hindlimbs adpressed; tibiotarsal articulation extends to posterior corner of eye; tarsal fold thick, extending to heel; inner metatarsal tubercle small and elliptical; outer metatarsal tubercle small, flat, and indistinct; subarticular tubercles small and round; single row of supernumerary tubercles on proximal segments of each digit; toes moderately short and slender; length of toes from shortest to longest, 1-2-3-5-4; toes fully webbed; flap of skin on inner surface of first toe; discs about same size as those on fingers. Anal opening directed posteroventrally at middle of thighs; anal sheath moderately elongate; small tubercles below anal opening. Skin of dorsum rather smooth, somewhat granular on dorsal surfaces of limbs; skin of chin and belly moderately granular; that of posterior surfaces of thighs smooth; no thoracic fold. Tongue nearly round, shallowly notched posteriorly, and free for about one-fourth its length; vomerine teeth 5-5, situated on rounded ridges between small inner nares; no vocal slits.
Color (in alcohol) dull olive-green on dorsal surfaces of head, body, and limbs; flanks dull olive-green with scattered cream-colored spots; posterior surfaces of thighs grayish brown with faint creamy mottling; chin gray with cream-colored spots; belly creamy yellow, suffused with gray posteriorly; undersides of feet and webbing gray; anal stripe faint, pale cream-color.
Variation.—The only other known specimen (MNHN 6331) is a female having a snout-vent length of 53.7 mm. and resembling the specimen described above in most details of morphology. In MNHN 6331 the tympanum is completely concealed, and the 8-7 vomerine teeth are arranged in two irregular rows. The female has more cream-colored mottling on the flanks and posterior surfaces of the thighs and more distinct mottling on the throat than the male described above.
Remarks.—The systematic status of Cauphias crassus Brocchi has been in doubt since the time of the original description. Brocchi (1877:130) stated: "Les dernieres phalanges sont obtuses, tronqués a leur extrémité antérieure." Brocchi placed the species in his genus Cauphias (type species, C. guatemalensis), which he considered to be related to Hylodes ( = Eleutherodactylus in the sense used by Brocchi); he thereby placed Cauphias in his Hylodidae ( = Leptodactylidae, in part). This idea of relationships was perpetuated by Barbour (1927:96), who reported on the second known specimen of Cauphias guatemalensis and stated: "When I dissected the sternum I was at once struck by its similarity to Noble's figures of transitional types between arciferal and firmisternal forms. The Cauphias sternum recalls some of his figures for Sminthillus and Eleutherodactylus. This genus is probably most closely related to the latter and has probably become highly modified to meet some peculiar environmental condition or on account of some specialized habits as yet unknown." Kellogg (1932:118) placed Cauphias in the Leptodactylidae and stated that the terminal phalanges are T-shaped. Hartweg (1941:1) considered Plectrohyla to be the correct generic name for Cauphias guatemalensis; he thereby relegated Cauphias to the synonomy of Plectrohyla. Hartweg (1941:9) further showed that the terminal phalanges of Plectrohyla guatemalensis were not T-shaped and that intercalary cartilages were present. Thus, he correctly concluded that Plectrohyla guatemalensis (and P. crassa by implication) was a member of the family Hylidae. Stuart (1942:6) followed Hartweg's allocations and further suggested that Plectrohyla crassa might be the same species as Hyla robustofemora Taylor. In his description of H. robustofemora Taylor (1940:392), who had not examined the type of Cauphias crassus, stated that were it not for the statements of Brocchi and Kellogg that C. crassus has T-shaped terminal phalanges, "I might suspect I had before me a specimen of Cauphias closely related to crassum."
I have compared the type of Cauphias crassus with that of Hyla robustofemora. With the exception of the minor differences mentioned in the preceding section on variation, the specimens are alike, leaving little doubt that they represent the same species. The statements of Brocchi and Kellogg to the contrary, the type of Cauphias crassus possesses intercalary cartilages between the penultimate and terminal phalanges; the latter are not T-shaped, but as in the type of Hyla robustofemora, resemble those typical of Hyla. On the basis of the morphological characters, as pointed out for Hyla robustofemora by Taylor (1940:392), Hyla crassa is a member of the Hyla bistincta group.
Distribution.—This species is definitely known only from a small stream at an elevation of 2300 meters in the mountains of central Oaxaca (Fig. 4).
Specimens examined.—Oaxaca: Cerro San Felipe, UIMNH 25050. "Mexico," MNHN 6331.
RELATIONSHIPS
The evolutionary trend in the members of the Hyla bistincta group is towards aquatic habits. Hyla bistincta, the least specialized species in the group, has relatively short fingers, webbing between the fingers, a truncate, high snout, and relatively large subarticular and supernumerary tubercles. Hyla charadricola resembles bistincta in having relatively short fingers, a slight amount of webbing, and a truncate snout. Apparently these two species are more closely related to one another than either is to the other species in the group. Hyla robertsorum, pachyderma, and crassa are the most aquatic members of the group. These species are closely related, possibly conspecific. All have round, sloping snouts, robust forearms, long, unwebbed fingers, and large webbed feet. Both H. pachyderma and H. crassa seem to be advanced beyond H. robertsorum. If small nuptial spines, moderately webbed feet, and absence of a well-defined thoracic fold are considered to be less advanced than large nuptial spines and a strong thoracic fold, as in H. pachyderma, or fully webbed feet, as in H. crassa, then H. robertsorum must be considered to be less advanced than H. pachyderma or H. crassa.
Members of the Hyla bistincta group inhabit mountain streams. The frogs can be found along these streams throughout the year. Since in most stream-breeding hylids there is no migration to breeding sites, the breeding call does not function to attract females to the breeding site. Apparently voices are lacking in all members of the Hyla bistincta group, except in Hyla bistincta. The presence of vocal slits and the ability to call further indicate that Hyla bistincta is the primitive member of this group.