Page:EB1911 - Volume 03.djvu/543

 consists of a plate of cartilage with two slender cornua, three processes on each side, and two long bony rods behind, termed the thyro-hyals, which embrace the larynx. In the Aglossa, which are remarkable for the large size and complexity of the larynx, the thyro-hyal bones are incorporated into the laryngeal apparatus, whilst the recently discovered Hymenochirus is further remarkable for the large size and ossification of the hyoidean cornua (ceratohyals), a feature which, though not uncommon among the salamanders, is unique among the Ecaudata (31).

The pectoral girdle of the Stegocephalia is, of course, only known from the ossified elements, the identification of which has given rise to some diversity of opinion. But C. Gegenbaur’s (32) interpretation may be regarded as final. He has shown that, as in the Crossopterygian and Chondrostean ganoid fishes, there are two clavicular elements on each side; the lower corresponds to the clavicle of reptiles and higher vertebrates, whilst the upper corresponds to the clavicle of teleostean fishes, and has been named by him “cleithrum.” As stated above, there is strong evidence in favour of the view that some forms at least possessed in addition a “supracleithrum,” corresponding to the supra-clavicle of bony fishes. The element often termed “coracoid” in these fossils would be the scapula. The clavicles rest on a large discoidal, rhomboidal, or T-shaped median bone, which clearly corresponds to the interclavicle of reptiles.

The pectoral girdle of the living types of batrachians is distinguishable into a scapular, a coracoidal, and a praecoracoidal region. In most of the Caudata the scapular region alone ossifies, but in the Ecaudata the coracoid is bony and a clavicle is frequently developed over the praecoracoid cartilage. In these batrachians the pectoral arch falls into two distinct types—the arciferous, in which the precoracoid (+ clavicle) and coracoid are widely separated from each other distally and connected by an arched cartilage (the epicoracoid), the right usually overlapping the left; and the firmisternal, in which both precoracoid and coracoid nearly abut on the median line, and are only narrowly separated by the more or less fused epicoracoids. The former type is exemplified by the toads and the lower Ecaudata, whilst the latter is characteristic of the true frogs (Ranidae), although when quite young these batrachians present a condition similar to that which persists throughout life in their lower relatives. A cartilage in the median line in front of the precoracoids, sometimes supported by a bony style, is the so-called Omosternum; a large one behind the cora-coids, also sometimes provided with a bony style, has been called the sternum. But these names will probably have to be changed when the homologies of these parts are better understood.

The pelvic arch of some of the Stegocephalia contained a well ossified pubic element, whilst in all other batrachians only the ilium, or the ilium and the ischium are ossified. In the Ecaudata the ilium is greatly elongated and the pubis and ischium are flattened, discoidal, and closely applied to their fellows by their inner surfaces; the pelvic girdle looks like a pair of tongs.

The long bones of the limbs consist of an axis of cartilage; the extremities of the cartilages frequently undergo calcification and are thus converted into epiphyses. In the Ecaudata the radius and ulna coalesce into one bone. The carpus, which remains cartilaginous in many of the Stegocephalia and Caudata, contains six to eight elements when the manus is fully developed, whilst the number is reduced in those forms which have only two or three digits. Except in some of the Stegocephalia, there are only four functional digits in the manus, but the Ecaudata have a more or less distinct rudiment of pollex; in the Caudata it seems to be the outer digit which has been suppressed, as atavistic reappearance of a fifth digit takes place on the outer side of the manus, as it does on the pes in those forms in which the toes are reduced to four. The usual number of phalanges is 2, 2, 3, 2 in the Stegocephalia and Caudata, 2, 2, 3, 3 in the Ecaudata. In the foot the digits usually number five, and the phalanges 2, 2, 3, 3, 2 in the Caudata, 2, 2, 3, 4, 3 in the Stegocephalia and Ecaudata. There are occasionally intercalary ossifications between the two distal phalanges (33). There are usually nine tarsal elements in the Caudata; this number is reduced in the Ecaudata, in which the two bones of the proximal row (sometimes coalesced) are much elongated and form an additional segment to the greatly lengthened hind-limb, a sort of crus secundarium. In the Ecaudata also, the tibia and fibula coalesce into one bone, and two or three small bones on the inner side of the tarsus form what has been regarded as a rudimentary digit or “prehallux.”

Integument.—In all recent batrachians, the skin is naked, or if small scales are present, as in many of the Apoda, they are concealed in the skin. The extinct Stegocephalia, on the other hand, were mostly protected, on the ventral surface at least, by an armour of overlapping round, oval, or rhomboidal scales, often very similar to those of Crossopterygian or ganoid fishes, and likewise disposed in transverse oblique lines converging forwards on the middle line of the belly. Sometimes these scales assumed the importance of scutes and formed a carapace, as in the “batrachian armadillo” discovered by E. D. Cope. A few frogs have the skin of the back studded with stellate bony deposits (Phyllomedusa, Nototrema), whilst two genera are remarkable for possessing a bony dorsal shield, free from the vertebrae (Ceratorphrys) or ankylosed to them (Brachycephalus). None of the Stegocephalia appears to have been provided with claws, but some living batrachians (Onychodactylus, Xenopus, Hymenochirus) have the tips of some or all of the digits protected by a claw-like horny sheath.

The integument of tailed and tailless batrachians is remarkable for the great abundance of follicular glands, of which there may be two kinds, each having a special secretion, which is always more or less acrid and irritating, and affords a means of defence against the attacks of many carnivorous animals. A great deal has been