Sub-Fam. 1. Engystomatinae.–Without teeth in the upper jaw.–Although there are only about 60 species known, these have been grouped into more than two dozen genera, many of which are represented by one or two species only. The range of this sub-family is peculiar, namely, Neotropical and Palaeotropical. Scaphiophryne and Rhombophryne are peculiar to Madagascar; Calophrynus occurs in the same island and in the Indian region; Xenobatrachus, Sphenophryne, Liophryne, Mantophryne, Callulops and Xenorhina live in New Guinea. Breviceps, Cacosternum and Hemisus are confined to Africa, while of Phrynomantis two species live in Africa, and the third in the Malay island of Amboina. Such freaks of distribution indicate either that many of these genera are not established upon very valid characters, or that their respective species are instances of convergent evolution, and do not form natural genetic groups.

Many of the members of this sub-family live upon ants and termites, and it is a well-known fact, not restricted to the Anura, that this kind of fare has a peculiar, modifying influence upon the structure of the mouth, teeth, tongue, limbs, and various other organs. In the present case the tongue is not much affected; it is, with few exceptions, more or less oval, not nicked, but free behind; in the Indian Glyphoglossus and in Rhombophryne of Madagascar only is it modified into a rather long and grooved, almost double, apparatus.

A very common feature is the small size of the mouth and the formation of a snout, which projects beyond the upper rim of the mouth and beyond the nostrils. Such a prominent and pointed snout is well developed in Rhinoderma, Phryniscus, Calophrynus, Stereocyclops, Hypopachus and Engystoma. The mouth is very narrow in Cacopus, Glyphoglossus, Breviceps, Rhombophryne, and Hemisus, all creatures which seem to be confirmed eaters of ants and termites. However, it must not be supposed that the mouth of all the genera is narrow, although this character, rather marked in Engystoma, is now embodied in the name of the family. A peculiar development of the palatal region is possibly correlated with this food. The palate is mostly toothless, but its skin is frequently raised into a transverse fold, between or behind the vomers, and into a second fold in front of the oesophagus; these folds are sometimes rather hard and serrated or denticulated. The palatine bones carry true teeth in Rhombophryne, and sometimes in Callula; in Xenobatrachus the teeth are reduced to two large pairs. The tympanum is usually hidden.

The shape of the body is generally very stout. The limbs are short, notably so in Glyphoglossus, Breviceps, Rhombophryne, Hemisus, Stereocyclops and Cacopus. Others, for instance most species of Microhyla, Phryniscus, Callula, and Sphenophryne, are of a very slender build; and their limbs, instead of being short and well adapted to digging, are long and may even be provided with typical adhesive discs, supported by T-shaped phalanges, especially in the two genera last named, and in Scaphiophryne and Phrynomantis. However, none of the forms provided with discs are known to be arboreal.

Exceptional diversity is shown in the shoulder-girdle and sternum. The omosternum occurs only in Rhinoderma and Hemisus. The metasternum is a cartilaginous plate, very large in Cacopus, distinctly small in Breviceps, and almost absent in Hemisus. The precoracoids and clavicles show all stages from a well-developed condition (Breviceps, Rhombophryne, Hemisus, Rhinoderma, Phryniscus and Brachycephalus) to complete absence. The circumstance that these bars are very weak in Melanobatrachus, Calophrynus, Scaphiophryne and Hypopachus, i.e. in Palæo- and Neo-tropical genera, indicates a widespread tendency towards complete suppression, a feature independently aimed at both in America (Engystoma) and in the Old World.

Until we know something about the habits of the members of this much diversified sub-family, it is idle to connect the various modifications with each other, and thus, by correlation, to find out their meaning. Those forms which possess well-developed discs on their fingers and toes are said not to be arboreal. What is the true meaning of the prominent snout which is not restricted to the digging forms? Most of the good diggers have well-developed precoracoid bars, and the coracoids are distinctly strengthened, but in Glyphoglossus and in Cacopus the precoracoids are entirely absent, and this loss is compensated for by exceptionally strong coracoids.

On the whole, those genera are to be considered as the most primitive which have undergone the fewest losses. Those with a complete shoulder-girdle, with an omo- and meta-sternum and with simple phalanges, are necessarily the older forms. One step farther back in another direction, the possession of teeth on the palate, and on the upper jaw, leads to those genera which have been separated off as Dyscophinae, while teeth in the lower jaw constitute the Genyophryninae. Lastly, the firmisternal type has necessarily been evolved from the arciferous condition, and there the two Bufonid genera Myobatrachus and Rhinophrynus, the former Australian, the latter Mexican, with their narrow and scarcely overlapping epicoracoid cartilages, seem to form a connecting link, although their ant-eating habits, with concomitant modifications in structure, may be nothing but cases of convergent evolution.

Key to the genera:–

I. American. A. with omosternum .......... Rhinoderma, p. [228].

B. without omosternum.