GENERAL STRUCTURE AND ORDERS. IOI9 



nate vertebral segments, and from circumstances to be detailed in 

 the sequel it is inferred by Professor Cope and others that the verte- 

 bral bodies of existing Amphibians are really intercentra, carrying 

 the neural arches which have been shifted to them from the lost 

 centra. The ribs may articulate to the vertebra by a single (upper) 

 transverse process ■ but in Salamandroids and Labyrinthodonts the 

 vertebrae also carry an inferior rib-facet, when the heads of the ribs 

 are consequently double. As a rule no ribless (lumbar) vertebrae 

 occur, and sternal ribs are wanting. The sternum, which is never 

 present in Fishes, appears in the Amphibia in the middle line of 

 the chest, and may be extended anteriorly as an omo- and epi- 

 sternum (fig. 974, b). The pectoral and pelvic girdles are well 

 developed ; the former (ibid.) in recent forms having, however, no 

 clavicle or interclavicle, although it is nearly certain that these bones 

 are represented in the thoracic buckler of the Labyrinthodonts. A 

 distinct rod-like precoracoid (fig. 974, b) occurs in recent forms. In 

 the pelvis the pubis is frequently unossified, and if ossified is much 

 smaller than the ischium. In most cases the ischium and the car- 

 tilaginous or bony pubis of either side unite to form a continuous 

 plate, in which the obturator foramen is not represented, although 

 there is a small perforation ; and the ischia meet in a ventral symphy- 

 sis. The body of the ilium may be almost wholly in advance of the 

 acetabulum. The limbs vary greatly in their proportionate length 

 in the different orders, and may be entirely absent ; while the carpals 

 and tarsals may be unossified. The limb bones of the Labyrinth- 

 odonts approximate to those of the Anomodont Reptiles. The 

 carpus and tarsus always have a centrale, and in some of the primit- 

 ive Labyrinthodonts there may be at least three centralia in the 

 latter. The fourth and fifth tarsalia always remain distinct from 

 each other in those forms which have five digits. The latter number 

 is found in the Labyrinthodonts, but in some existing forms the 

 digits may be reduced to three or two. The number of the 

 phalangeals in the digits of pentedactylate forms, counting from 

 the first to the fifth, does not exceed 2, 2, 3, 3, 3, and this num- 

 ber may be reduced in some existing forms. As a rule the tail 

 is comparatively short. 



Teeth are usually present on the premaxilla, maxilla, vomer, and 

 the dentary bone of the mandible ; but are generally wanting on 

 the palatine and pterygoid, although present on the latter, and also 

 on the parasphenoid, in many Labyrinthodonts. These teeth are 

 usually anchylosed to the bone, and in existing forms are of simple 

 structure. In the Labyrinthodonts the structure of the teeth may, 

 however, become very complex by foldings of the dentine, this 

 structure being an extreme development of that met with in certain 

 Ganoids ; and in some of these forms the usually large pulp-cavity 



