116 AMPHIBIA CLASsii 



size, and enter into the border of the tympanic region. The latter is roofed 

 by several small bones corresponding to the proötic and opisthotic of fishes, or 

 these may unite into a single element called the petrosal. The antero-lateral 

 walls of the skull remain cartilaginous ; but in the ethmoidal region an ossifi- 

 cation takes place, forming the orbitosphenoid which is usually separate, but 

 sometimes {Anura) fuses with the median elements to form a single ring- 

 shaped bone (sphen-ethmoid). 



The cranial roof is formed by the paired parietals, frontals, pre- and post- 

 frontals, and nasals; and in Stegocephalians there are present in addition the so- 

 called supratemporals, squamosals, postorbitals, and lachrymals. The palate is 

 formed as in fishes by a large median parasphenoid, and usually paired vomers, 

 and palatines. There is no movable Suspensorium for the lower jaw ; the 

 cartilage representing it unites with the squamosal above, and quadrato-jugal 

 below. Sometimes an ossified quadrate is formed at the end of the suspen- 

 sorial cartilage. Attached to the quadrato-jugal in front are the maxillae, 

 and anterior to these the premaxillae, which complete the rim of the upper jaw 

 in front. Many of the Urodeles have the maxillae and quadrato-jugal replaced 

 by connective tissue. Between the quadrate and parasphenoid is placed the 

 pterygoid, usually a trifid bone which joins the parasphenoid by its shorter 

 arm, while its anterior brauch forms the outer border of the palatal vacuity. 

 The palatines, when present, usually join the anterior ends of the pterygoids, 

 and extend parallel with the maxillae. The mandibular ramus is composed 

 of three or four elements, as in fishes. The visceral skeleton is formed by 

 the paired hyoid bones, and, in gill-breathers, by three or four partially ossi- 

 fied branchial arches. 



The teeth are acutely conical, and are commonly borne by the mandible, 

 maxillae, premaxillae, vomer, and palatines. Earely the parasphenoid and 

 pterygoid are armed with minute teeth, and only certain Anura are edentulous. 

 Teeth of the acrodont type have their bases implanted directly upon the 

 rim or top of the jaws ; those of the pleurodont type are sunk against the 

 inner side of the jaws. As in Ganoids and bony fishes, worn teeth are not 

 replaced by successional ones developed beneath them, but new ones are 

 formed independently alongside the old, and gradually oust the latter as their 

 basal parts become reabsorbed. Amphibian teeth difl'er from those of fishes 

 chiefly in the absence of vasodentine ; and the spacious pulp cavity which 

 during life contains vascular or connective tissue, occurs in the fossil State 

 either hollow or impregnated with mineral matter. Some Stegocephalians 

 have complex or " labyrinthodont " teeth, the dentine being strongly folded, 

 as in certain Crossopterygian fishes. 



In the pedoral arch the scapula is ossified in at least its proximal portion, 

 where it joins the coracoid and pre-coracoid, and forms the articular face 

 for the humerus. A bony sternum is usually absent. Stegocephalians 

 are peculiar in possessing between the pectoral limbs one median and two 

 paired exoskeletal plates, commonly regarded as interclavicle and clavicles. 

 The fore-limb is composed of the usual bones, humerus, radius, and ulna ; a 

 carpus which is either cartilaginous or consists of two rows of ossicles ; and 

 three, four, or five metacarpals which support digits with from one to four 

 phalanges. 



The pelvic arch is composed of a long and slender bony ilium, which is 

 attached either directly to the transverse process of the sacral vertebra, or to 



