Teeth are almost entirely absent, except in Notaden, which has teeth on the vomers. The omosternum is mostly absent, except in Engystomops and in some species of Bufo, while in Notaden it is merely vestigial. The metasternum shows more variety. The tympanum is usually distinct, but varies even within the same genus, being hidden beneath the skin or being entirely absent. The terminal phalanges are modified according to the habits of the species, but they are never claw-shaped.
The Bufonidae are connected in various directions. The Neotropical Engystomops greatly resembles the likewise Neotropical Cystignathoid Paludicola, and the Australian Pseudophryne closely approaches the Australian Cystignathoid Crinia. It is therefore all the more remarkable that a similar approach, in another direction, namely, towards the Firmisternal family of the Engystomatidae, is indicated by the Mexican Rhinophrys and the Australian Myobatrachus. However, since there are no true Engystomatidae in Australia, although several genera occur in Papuasia, these cases may be instances of convergence without necessarily implying relationship. An unmistakable line of connexion leads, according to Boulenger, to the Pelobatidae, the link being the Himalayan Cophophryne, with very strongly dilated sacral diapophyses, with a single condylar articulation of the coccyx with the sacral vertebra (as in some Indo-Malayan Pelobatidae), while this articulation is bicondylar in all the other Bufonidae.
Fig. 34.–Map showing distribution of Bufonidae. The vertical lines indicate the occurrence of Bufonidae, but not of Bufo.
The whole family is divided into nine genera with more than a hundred species, of which only about fifteen do not belong to the genus Bufo. The distribution of the family is well-nigh cosmopolitan, with the remarkable exception of Madagascar, Papuasia, and the small islands of the Pacific; Bufo has been wrongly said to inhabit the Sandwich Islands. The greatest number of species, chiefly Bufo, occur in the Neotropical region, the greatest number of genera in Central America, where Bufo is rare, and in Australia, where it is absent.
A. Pupils contracted to a horizontal slit. Typically arciferous.
a. Australian. Tympanum invisible. Fingers and toes not dilated.
1. With vomerine teeth. Both the omo- and meta-sternum are rudimentary. East Australia: .......... Notaden bennetti.
2. Without vomerine teeth. Omosternum absent. Metasternum cartilaginous: .......... Pseudophryne, p. [168].
b. Not Australian.