Fam. 5. Cystignathidae.–This is one of the largest families, and also one of the least satisfactory. Its numerous members, more than 150, exhibit such a versatility in adaptation to circumstances (there are aquatic, terrestrial, arboreal, and burrowing species), with a corresponding development or loss of anatomical characters which we should like to rely upon as taxonomic marks, that the numerous genera not only run into each other, but also get entangled with those of other families. In fact the whole family is ill defined. It can be characterised as follows:–The shoulder-girdle is arciferous; the sacral diapophyses are cylindrical or but slightly dilated; the metasternum has either a bony style or it forms a cartilaginous plate; the terminal phalanges, although they sometimes carry adhesive discs, are never claw-shaped.
The last statement is, of course, intended to separate the Cystignathidae from the Hylidae, of which, however, the three genera Thoropa, Chorophilus, and Acris stand on debatable ground (cf. p. [186], Hylidae), while, on the other hand, most of the Australian genera, notably Chiroleptes, have unmistakably dilated sacral diapophyses. The difference from the Pelobatidae can in this case be one of degree only.
The Cystignathidae may be said to represent the Ranidae in Notogaea. Some of them can be distinguished from the true, typical frogs solely by the arciferous type of the shoulder-girdle and sternum. There is in both families the same adaptive versatility, the same amplitude in the formation of the fingertips, the occasional slight dilatation of the sacral diapophyses, the same range in the configuration of the omo- and meta-sternum. In fact, young Ranidae, before the firmisternal character is assumed, are indistinguishable from Cystignathidae, and the latter would turn into Ranidae if they could be induced to consolidate their sternal apparatus.
The geographical distribution of the Cystignathidae is suggestive of their being an old family, most of whose members have reached a high stage of morphological development. The overwhelming majority inhabit the Neotropical region, a few forms extending into tropical Central America and into the Antilles; the rest, some twenty species only, are confined to the Continent of Australia and to Tasmania.
The family name is rather a misnomer. It is taken from the genus Cystignathus, which is, or rather was, characterised by the peculiarly broadened lower jaw, hollowed out by the vocal sacs; but this generic name had to give way to that of Leptodactylus, in obedience to the often senseless rule of priority. The family is composed of three subfamilies.
Sub-Fam. 1. Hemiphractinae.–Teeth are carried by both jaws, the vomers and the palatine bones; or by the palatines and parasphenoids in Amphodus. The vertebrae are opisthocoelous, devoid of ribs, and the sacral diapophyses are not dilated. The shoulder-girdle and sternum are strictly arciferous. The omosternum is very much reduced; the metasternum forms a cartilaginous plate. The tongue is slightly free behind. The tympanum is distinct. Three genera, with eight species, all inhabitants of South America.
Hemiphractus.–The head is large; the upper surface of all the cranial bones appears pitted, owing to most of the covering skin being involved in the ossification. The temporal fossa is bridged over or roofed in by the fronto-parietal and the squamosals, so that the orbit is completely encircled by bone, as in Pelobates cultripes. The terminal phalanges are simple and are not dilated into discs. The teeth of the lower jaw are very small and numerous. The tongue is round and very small. H. scutatus, the only species, living in Ecuador and Colombia, is a frog-like creature, with a large helmet-shaped head. Total length 2½ inches.
Ceratohyla has the same kind of helmet-shaped head, and the orbit is likewise enclosed by bone, but the terminal phalanges are claw-shaped and carry regular adhesive discs. This genus, the five species of which live in Ecuador, bears undoubted resemblances to the Hylidae. In C. proboscidea the upper eyelid is produced into a little upright fold, as in Amphignathodon and some species of Nototrema and Ceratophrys among Cystignathidae. The snout is produced into a long, compressed, bifid appendage, and the heel carries a triangular flap. In C. bubalus the partly ossified helmet sends out a pair of diverging processes, formed by the squamosals, extending backwards and sideways from the concave and ridged interorbital spaces. The tip of the snout and the tips of the divergent horns form an equilateral triangle, and the whole head bears a striking resemblance to some of the fossil Reptiles from the Elgin Sandstone, e.g. Triceratops. Total length 3 inches.
Amphodus wucheri.–The only species of this genus has been found near Bahia. It has teeth on the palatine bones and five series of small teeth on the parasphenoid, but none on the vomers. The teeth of the mandible number about eleven on each side and decrease in size towards the symphysis. The tympanum is distinct; the heart-shaped tongue is free behind. The cranial bones are only slightly pitted. The skin is smooth above, chocolate-brown, spotted with yellow, and with a yellow band on the sides of the body beginning with the upper eyelid and ending in a broad patch above the vent. The under parts are yellowish white.
Sub-Fam. 2. Cystignathinae.–The upper jaw alone is provided with teeth. Vertebrae procoelous. The twenty-seven genera of this sub-family have been arranged in the following key, merely for convenient determination.