514 



CHORDATA 



The vertebral column often (Perennibranchs, Derotremes, Cascilians, 

 and many Stegocephali) resembles that of fishes in amphiccelous centra 

 and persistence of notochord. The notochord may disappear, there then 

 occurring opisthoccelous (Salamandrina) or precocious centra (most 

 Anura). There is also an articulation of skull with vertebral column, 

 rare in fishes but characteristic of land animals, by which the first 

 vertebra (atlas) becomes distinct from the rest. 



The skull is remarkable for the extent to which the chondrocranium 

 is retained and the consequent small number of primary bones (figs. 565, 

 566). The bones of the orbital region are represented by a pair each of 

 ali- and orbitosphenoids in the urodeles, by a ring of bone, ihesphenet/nnoid, 

 in the anura. The auditory region usually contains only prootics, the 



TYL 



C'o'cc 



FIG. 565. Frog skull from below (from Wiedersheim). For letters see fig. 566. 



occipital only exoccipitals. The absence of other occipitals is often of 

 value in distinguishing between amphibian and reptilian skulls, since in 

 the former the articulation with the atlas is consequently by double 

 oa. i filial condyles. Of secondary cranial bones are to be mentioned the 

 nasals, f rentals (in many prefrontals also), and parietals, the latter two 

 fused in anura to Jronto parietals; ventrally the large parasphenoid. 



The cranium is increased by the addition of the large quadrate cartilage, 

 which becomes applied to the otic capsule and (Anura) fuses with it, while the 

 rest of its arch (pterygoid) extends forward in a more or less complete condition, 

 reaching the nasal capsule in the Anura. The quadrate cartilage is covered 

 externally by the squamosal and supports the lower jaw, composed of Meckel's 

 cartilage surrounded by membrane bones (dentary, splenial, angulare, etc.); its 



