Calling males were found in and about ponds on the savannas in southern Veracruz, in ponds in open forest in northern Oaxaca (not in forest pools), and in temporary pools in the scrub forest on the Pacific lowlands. Individuals usually called from bushes and reeds in or at the edge of ponds. The call is a short "braaa." Dates of breeding choruses indicate that by the time the other small species of hylids in the Gulf lowlands reach the peak of their breeding season, that of _H.

staufferi_ is essentially over; no large breeding congregations were found in July. On July 8, 1956, two metamorphosing young were found clinging to blades of gra.s.s in a pond; they had snout-vent lengths of 8 and 9 mm. and tail stumps less than 3 mm. in length. Others were found on July 13 and 26. The juveniles are nearly unicolor olive green above and white below.

In life the adults vary greatly in color pattern. The dorsal ground color is yellowish tan to olive brown with olive brown or dark brown spots, some of which in certain individuals are connected to form longitudinal dark stripes. On the posterior surface of the thighs are small white flecks. The belly is white, and the vocal sac is a rich yellow. Twenty males have an average snout-vent length of 26.3 mm.; they have no h.o.r.n.y nuptial pads. No noticeable differences in either color or body proportions were found between the populations on either side of the isthmus.

=Hylella sumichrasti= Brocchi

_Oaxaca_: Cerro Arenal (5); Cerro San Pedro (2); Escurano; La Concepcion (41); Portillo Los Nanches (6); San Antonio (16); 11 km. S of Santiago Chivela (18); Santa Lucia (7); Tapanatepec (5); Tehuantepec (8); Tenango (49); Tres Cruces (19).

With the exception of the series from 11 kilometers south of Santiago Chivela, most of these specimens were found in small arboreal bromeliads during the dry season. Males were found along a clear, shallow, rocky stream south of Santiago Chivela on July 6, 1956. The frogs were calling from bushes and rocks in and along the stream. When disturbed, they jumped into the water and floated downstream until they were able to hold onto a rock or other object. The call is a loud "bra-a-ah." In breeding individuals the dorsum is pale yellow; the belly is white, and the vocal sac is yellow. The iris is pale golden yellow. Eighteen males have an average snout-vent length of 25.2 mm.

All have dark brown nuptial tuberosities on the pollex.

Certain diagnostic characters of this species as given by Taylor (1943a:50) and Taylor and Smith (1945:598) are in need of revision.

_Hylella sumichrasti_ has been characterized as having no vocal sac, rarely having vomerine teeth, and as having a relatively smooth throat.

The vocal sac in breeding males is quite evident; it is single, median, and when expanded, spherical. The openings into the vocal sac are narrow slits along the inner posterior border of the jaw rami. Of 151 specimens studied, 74 have vomerine ridges between the choanae, and 36 of these have one to three teeth on each ridge. The belly and undersurfaces of the thighs are granular; the throat is only somewhat less so. The granular condition may be correlated with breeding, for specimens obtained from bromeliads in the dry season had rather smooth throats. It seems that the vocal sac atrophys in the non-breeding season. These seasonal changes may account for the diagnoses given by Taylor (_op. cit._) and Taylor and Smith (_op. cit._); likewise, since many of the specimens obtained by Smith in the dry season were juveniles and subadults, the development of the vomerine ridges could not be diagnosed properly.

The range of this species encompa.s.ses the Pacific slopes of the Isthmus of Tehuantepec eastward to the upper Cintalapa Valley and vicinity of Tonala in western Chiapas. Priscilla Starrett collected tadpoles of _H.

sumichrasti_ from a stream 19 km. N of Arriaga, Chiapas. These limited observations on the ecology of this frog suggest that it breeds in the fast-moving streams of the Pacific slopes, and that it seeks shelter in arboreal bromeliads during the dry season.

=Phrynohyas modesta= Taylor and Smith

_Oaxaca_: Tuxtepec. _Veracruz_: 20 km. S of Jesus Carranza; 20 km. ENE of Jesus Carranza (2); Minat.i.tlan.

I have not collected this species in the isthmus. The locality records indicate that the range is discontinuous (Duellman, 1956:27). The species occurs on the humid Pacific slopes from south-central Chiapas eastward to El Salvador and on the humid Gulf lowlands from southern Veracruz eastward into Tabasco, but is unknown from the dry Pacific slopes and plains in the isthmus.

The acquisition of several specimens of this species in southern Veracruz, Tabasco, and Oaxaca, together with a knowledge of the variation displayed by _Phrynohyas spilomma_, suggests that _modesta_ may be a color variety of _spilomma_.

=Phrynohyas spilomma= Cope

_Oaxaca_: Tapanatepec (3). _Veracruz_: Amat.i.tlan (12); Chacaltianguis (2); Ciudad Aleman (6); Cosamaloapan; Novillero (3).

Like the preceding species, this frog is unknown from the arid Pacific lowlands of the isthmus; its presence at Tapanatepec, a locality situated in more mesic conditions than prevail on the Plains of Tehuantepec, indicates that it may have a distribution on the Pacific slopes much like that of _P. modesta_. Furthermore, this frog was not detected in the rainforests of the Gulf lowlands; in that region it was found only in scrub forest and savanna.

On July 26, 1956, numerous choruses of these frogs were heard between Ciudad Aleman and Tlacotalpan, Veracruz. The call is a loud, nasal "grawl" repeated continuously. The males call from the water. Several clasping pairs were found in shallow gra.s.sy ponds amidst the scrub forest. The ground color varies from reddish brown to tan with dark brown dorsal markings. The iris is golden, and the vocal sacs are dark olive brown. After a light shower during the dry season, six individuals were found on the low branches of trees at night near Ciudad Aleman.

=Phyllomedusa callidryas taylori= Funkhouser

_Oaxaca_: Donaji (9); Sarabia (8); Tolosita (6); Ubero (27).

_Veracruz_: Alvarado (7); Aquilera; Berta; Coatzacoalcos (9); 10 km. SE of Hueyapan (5); Naranja (17).

In life this frog presents a striking array of colors. The dorsum varies from pale green to dark olive green; there may be scattered whitish or cream-colored spots on the back. On the flanks are bright yellow to deep cream-colored vertical bars separated by pale blue or purple inters.p.a.ces. The thighs and undersurfaces of the hind limbs are golden orange; the belly is yellow, and the throat is cream-colored.

The iris is crimson; the transparent part of the lower eyelid has golden reticulations. When the frog is resting, the forefeet are folded beneath the throat, and the limbs are folded tightly against the body.

In this position and with the eyes closed and head flattened, this gaudy frog a.s.sumes the appearance of a small elliptical green leaf.

Throughout the month of July, 1956, _Phyllomedusa_ was breeding in ponds in or adjacent to the rainforest in northern Oaxaca and in southern Veracruz. Only at Alvarado was it found breeding in a gra.s.sy pond. Males and females alike were found on bushes and trees in and around the ponds. The call is a single "w.a.n.k." Amplexing males continue to call, but the call is softer and less nasal in quality. The eggs are encased in pale green gelatin and attached to leaves on branches overhanging the water. Three egg clutches contained 38, 41, and 46 eggs.

=Phyllomedusa dacnicolor= Cope

_Oaxaca_: Escurano; Tehuantepec.

Although it is abundant on the Pacific lowlands to the northwest in Guerrero, Michoacan, and Colima, this species is known only from two specimens from Tehuantepec. There is no apparent physical barrier to their distribution in the isthmus; in the Balsas Basin the species lives in a hotter, more arid environment than that at Tehuantepec.

=Gastrophryne usta= Cope

_Oaxaca_: Santa Efigenia; Tehuantepec (10); 24 km. W of Tehuantepec; Tolosita (2). _Veracruz_: Ayentes (6); La Oaxaquena; Novillero (2); San Lorenzo.

Calling males were found in open scrub forest near Tehuantepec and in savannas near Novillero. The specimens from Tolosita were found under cover in a clearing in the forest (Fugler and Webb, 1957:106).

Specimens from the Pacific lowlands are typical of _Gastrophryne usta gadowi_ Boulenger in possessing a thin line on the posterior surface of the thighs and a thin line from the snout to the vent. Of nine specimens from the Gulf lowlands (Ayentes, Novillero, and San Lorenzo), seven have a middorsal line; this is narrow in four and wide in three.

Five have the stripes on the thighs. Two specimens from the middle of the isthmus (Tolosita) have no stripes on the thighs; one has a thin middorsal line, and the other has a broad line. The adult males have a black throat; females have a mottled one. The brown reticulations on the bellies of specimens from the Gulf lowlands is bolder than on specimens from the Pacific lowlands. The presence of certain characters supposedly diagnostic of the subspecies _gadowi_ (line on dorsum and thighs) in the population of _usta_ in southern Veracruz suggests that a redefinition of the ranges of these subspecies will be in order when sufficient material is available to delimit them accurately. For the present I prefer to consider all specimens from the isthmus solely as _Gastrophryne usta_ without referring them to subspecies.

=Rana palmipes= Spix

_Oaxaca_: Matias Romero (11); 11 km. S of Santiago Chivela; Santo Domingo; Sarabia. _Veracruz_: Coatzacoalcos; Cuatotolapam; 25 km. SE of Jesus Carranza (4); Tlacotalpan (2); Tula.

Adults were found along streams and in marshes in savannas and rainforest. These frogs are wary and difficult to capture, even at night. _Rana palmipes_ is another species that has a discontinuous distribution in the isthmus. The species does not occur on the Pacific lowlands of the isthmus, but does occur on the more humid Pacific slopes of Chiapas and Guatemala.

Tadpoles were found in a small sluggish tributary to the Rio Sarabia.

=Rana pipiens= Schreber

_Oaxaca_: Agua Caliente; Cerro Quiengola; Escurano (14); Rio Sarabia (2); Tapanatepec (5); Tehuantepec (24). _Veracruz_: Acayucan; Cuatotolapam (15); Jesus Carranza (2); 20 km. S of Jesus Carranza (11); 25 km. SE of Jesus Carranza; 20 km. ENE of Jesus Carranza (10); San Lorenzo (10).

As in most other places in Mexico and northern Central America, this species occurs wherever there is permanent water. Males were heard calling from woodland ponds and from savanna ponds.

SUMMARY

Investigations of the amphibians and their environments in the Isthmus of Tehuantepec have been presented with the aim of gaining an understanding of the present biological and of the historical events responsible for the present patterns of distribution of amphibians in this region.

The Isthmus of Tehuantepec embraces three major environments--savanna, semi-arid scrub forest, and quasi-rainforest. The rainforest presents an environment noticeably different from the other two and has a different amphibian fauna.

a.n.a.lysis of present patterns of distribution shows that certain species are restricted to the rainforests on the Gulf lowlands; others live only in the semi-arid scrub forests on the Pacific lowlands. A third group of species lives on both the Gulf and Pacific lowlands; most of these species occur only in the scrub forests or savannas on the Gulf lowlands, but some also inhabit the rainforest. In one way or another the isthmus presents a barrier to the distribution of 75 per cent of the species of amphibians living in the lowlands; it is a greater barrier still to the species inhabiting the highlands on either side.

Present patterns of distribution are attributed to bioclimatic fluctuation in the Pleistocene. In the course of these climatic shifts, tropical environments and their amphibian inhabitants seem to have survived in the isthmian region.

The amphibian fauna of the lowlands of the Isthmus of Tehuantepec consists of 16 genera and 36 species. Systematic studies of all available specimens from the region show that _Eleutherodactylus conspicuus_ Taylor and Smith is a synonym of _Eleutherodactylus alfredi_ Boulenger and that _Hyla axillamembrana_ Shannon and Werler is a synonym of _Hyla loquax_ Gaige and Stuart.