CHAPTER XII.
GENERAL ACCOUNT OF THE SKELETON IN AMPHIBIA.
EXOSKELETON.
The exoskeleton, at any rate in most living forms, is very slightly developed in Amphibia. The only representatives of the epidermal exoskeleton are (1) the minute horny beaks found coating the premaxillae and dentaries in Siren and the tadpoles of most Anura, (2) the nails borne by the first three digits of the pes in Xenopus and by the Japanese Salamander Onychodactylus, (3) the horny covering of the calcar or prehallux of frogs. The Urodela and nearly all the Anura, which form the vast majority of living Amphibia, have naked skins. A few Anura belonging to the genera Ceratophrys and Brachycephalus have bony dermal plates developed in the skin of the back, and these plates become united with some of the underlying vertebrae.
In the Gymnophiona the integument bears small cycloid scales arranged in rings which are equal in number to the vertebrae. These scales contain calcareous concretions. Scales also occur between the successive rings.
In the Labyrinthodontia the dermal exoskeleton is in many genera greatly developed. It is generally limited to the ventral surface and consists principally of a buckler formed of three bony plates, one median and two lateral. These plates protect the anterior part of the thorax, and are closely connected with the adjacent endoskeleton. They probably represent the interclavicle and clavicles. Behind this buckler numerous scutes are generally developed, which often cover the whole ventral surface, and may cover the whole body.
Teeth[59].
In Amphibia teeth are generally present on the maxillae, premaxillae and vomers, and except in Anura on the dentaries; sometimes they occur on the palatines as in many Urodela, most Labyrinthodontia, and the Gymnophiona; less commonly on the pterygoids as in Menobranchus, Siredon, some Labyrinthodontia, and Pelobates cultripes[60], or on the splenials as in Siren and Menobranchus, or parasphenoid as in Pelobates cultripes, Spelerpes belli and Batrachoseps. In some Anura such as Bufo and Pipa the jaws are toothless.
In Gymnophiona, Menobranchus, and Siredon, the teeth are arranged in two concentric curved rows. The teeth of the outer row are borne on the premaxillae and maxillae if present, (the maxillae are absent in Menobranchus), the teeth of the second row on the vomers and pterygoids in Menobranchus and Siredon, and on the vomers and palatines in Gymnophiona. In some Gymnophiona there is a double row of mandibular teeth. The vomerine, palatine and parasphenoid teeth of all forms are numerous and are not arranged in rows.
The teeth of all living Amphibia are simple conical structures ankylosed to the bone, and consisting of dentine, coated or capped with a thin layer of enamel. In the Labyrinthodontia teeth of more than one size are sometimes present. The dentine of the basal part of the larger teeth is in some genera very greatly folded, causing the structure to be highly complicated. These folds, the intervals between which are filled with cement, radiate inwards from the exterior and outwards from the large pulp cavity. The basal part of the teeth of Ceratophrys (Anura) has a similar structure.