This species inhabits the scrub forests of the Pacific coastal plain and the savannas in southern Veracruz; apparently it does not occur in rainforest. Consequently, its distribution in the isthmus is discontinuous.

PLATE 1

[Ill.u.s.tration: FIG. 1. Savanna about 75 kilometers east of Coatzacoalcos, Veracruz. Photograph by L. C. Stuart.]

[Ill.u.s.tration: FIG. 2. Low scrub forest near Alvarado, Veracruz.

Photograph by L. C. Stuart.]

PLATE 2

[Ill.u.s.tration: FIG. 1. Rainforest near Tolosita, Oaxaca. March, 1956.]

[Ill.u.s.tration: FIG. 2. Rainforest along the Rio Sarabia, Oaxaca. March, 1956.]

PLATE 3

[Ill.u.s.tration: FIG. 1. Transition forest near La Princesa, Oaxaca.

March, 1956.]

[Ill.u.s.tration: FIG. 2. Palm Savanna on the Plains of Chivela, Oaxaca.

March, 1956.]

PLATE 4

[Ill.u.s.tration: FIG. 1. Scrub forest on the Plains of Tehuantepec in dry season. March, 1956.]

[Ill.u.s.tration: FIG. 2. Scrub forest on the Plains of Tehuantepec in rainy season. View toward the north. In the distance is the Continental Divide in the hills of the Isthmus. July, 1958.]

PLATE 5

[Ill.u.s.tration: FIG. 1. Low, dense scrub forest near La Ventosa, Oaxaca.

July, 1958.]

[Ill.u.s.tration: FIG. 2. Temporary pond in scrub forest north of Salina Cruz, Oaxaca. July 7, 1958. _Rhinophrynus dorsalis_, _Bufo marmoreus_, and _Diaglena reticulata_ were breeding here the previous night.]

PLATE 6

[Ill.u.s.tration: FIG. 1. Calling male of _Rhinophrynus dorsalis_, photographed in a pond north of Santa Cruz, Oaxaca, on July 6, 1958.

2/3.]

[Ill.u.s.tration: FIG. 2. Color pattern variation in two adults of _Bufo ca.n.a.liferus_ from Juchitan, Oaxaca. 2/3.]

PLATE 7

[Ill.u.s.tration: FIG. 1. Calling male of _Engystomops pustulosus_, photographed in a pond west of Tehuantepec, Oaxaca, on July 5, 1956.

2.]

[Ill.u.s.tration: FIG. 2. Foamy egg ma.s.s of _Engystomops pustulosus_ at the edge of a pond west of Tehuantepec, Oaxaca. July 5, 1956. 3/8.]

PLATE 8

[Ill.u.s.tration: FIG. 1. Calling male of _Diaglena reticulata_, photographed at a pond north of Salina Cruz, Oaxaca, on July 6, 1958.

1/2.]

[Ill.u.s.tration: FIG. 2. Clasping pair of _Diaglena reticulata_ at the edge of a pond north of Salina Cruz, Oaxaca, on July 6, 1958. 1.]

Breeding congregations were found after heavy rains at Tehuantepec on July 5, 1956, at Cosamaloapan, Novillero, and Amat.i.tlan on July 26, 1956, and at Salina Cruz on July 6, 1958. The call is a long "worrp"

made while the male is floating on the surface of the pond. The small heads, small limbs, and greatly inflated bodies cause the calling males to resemble miniature caricature balloons (Pl. 6, fig. 1). Amplexus is inguinal. These toads are notably wary, even when calling. Often the beam of a flashlight or the slightest disturbance of the water will cause the males to stop calling. The body is deflated with one last nauseous note, and the frog sinks beneath the surface of the water and swims away with short slow kicks of the hind feet.

=Bufo ca.n.a.liferus= Cope

_Oaxaca_: Chivela; Salina Cruz; Santa Efigenia; Tapanatepec (6); Tehuantepec (10); Zanatepec (4).

This small toad apparently is restricted to the Pacific lowlands from the Isthmus of Tehuantepec eastward to Guatemala. At Zanatepec on July 13, 1956, males were calling from a flooded field bordered by scrub forest. The call is a rather loud nasal racket. Living individuals vary greatly in coloration. Some have yellowish tan flanks and dorsum and an orange middorsal stripe; others have a pale red dorsum, yellow flanks, and a cream middorsal stripe (Pl. 6, fig. 2).

=Bufo coccifer= Cope

_Oaxaca_: Juchitan (5); Tehuantepec.

It is with some degree of hesitancy that these toads are referred to the species _coccifer_. Although these and other specimens from Guerrero and Michoacan display no striking differences from specimens from Costa Rica, Nicaragua, and southeastern Guatemala, the ranges of the populations are separated by a broad hiatus in Chiapas and Guatemala. Possibly this species has utilized the sub-humid corridor through northern Central America (Stuart, 1954) and subsequently disappeared from the corridor in Guatemala and Chiapas. Specimens of a _coccifer_-like toad collected by Stuart in the vicinity of Jacaltenango, Departamento Huehuetenango, Guatemala, are much larger than either the Central American or Mexican specimens of _coccifer_. A final commitment on the systematic status must await a thorough study of this group of toads.

Males of this species were calling from a gra.s.sy rain-pool in open scrub forest at the edge of Juchitan on July 6, 1956. The call is a low "whirrr." The calling males were sitting in the shallow water at the edge of pond, where they were hidden by the gra.s.s. None was observed in open water, as is characteristic of calling males of _Bufo ca.n.a.liferus_ and _marmoreus_.

=Bufo marinus= Linnaeus

_Oaxaca_: Agua Caliente; Guichicovi (3); Mixtequilla; Tolosita (6); Tehuantepec (37); Tuxtepec; Union Hidalgo.

_Veracruz_: Ciudad Aleman (4); Cosamaloapan; Cuatotolapam (19); 20 km. SE of Jesus Carranza (4); 38 km. SE of Jesus Carranza (10); 20 km. NE of Jesus Carranza (4); Novillero.

This large toad is abundant throughout the lowlands of the isthmus. The loud rattling call of males was heard on rainy nights throughout the summer. In March, 1956, several adults were found in a small cave back of a spring at Agua Caliente.

=Bufo marmoreus= Wiegmann

_Oaxaca_: Cerro San Pedro (2); Chivela (5); Escurano (3); Juchitan; Salina Cruz (101); Santa Lucia (2); 12 km. S of Santiago Chivela (11); Santo Domingo; Tapanatepec; Tehuantepec (100); Tequisistlan. _Veracruz_: Alvarado; Coatzacoalcos.

This toad is abundant on the Pacific lowlands, where it inhabits both open and dense scrub forest. On the Gulf lowlands its distribution seems to be limited to xeric coastal habitats. Aside from the specimens from Alvarado and Coatzacoalcos, it is known in Veracruz only from Boca del Rio (Langebartel and Smith, 1959:27).

The similarity in size of _Bufo marmoreus_ and _valliceps_ and their almost completely allopatric ranges suggest that the two species may be in compet.i.tion at any one locality. Nevertheless, both were calling from a small rocky stream south of Santiago Chivela on July 6, 1956.

On the night of July 6, 1958, an estimated 400 toads of this species made up a breeding congregation near Salina Cruz. The site was a shallow muddy pond about 20 40 meters located in an area cleared of scrub forest; the banks of the pond were devoid of vegetation (Pl. 5, fig. 2). Breeding in the same pond were _Rhinophrynus dorsalis_ and _Diaglena reticulata_. The following morning no more than a dozen _Bufo_ were found in the pond, but several individuals were found beneath debris and in small burrows near the pond. On July 7, 1958, large numbers of tadpoles and recently metamorphosed young were in a shallow gra.s.sy pool just east of Salina Cruz.

Taylor (1943b:347) referred certain specimens from Tehuantepec to _Bufo perplexus_, a species closely related to _Bufo marmoreus_. Evidence to be presented elsewhere shows that _perplexus_ does not occur in the isthmus.

=Bufo valliceps valliceps= Wiegmann