IV. VERTEBEATA: AMPHIBIA. 



581 



or procoelous centra (most Annra). There is also an articulation 

 of sknll 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 chondrocra- 

 nium is retained and the consequent small number of primary 

 bones (figs. 611, 612). The bones of the orbital region are repre- 



PP 



TIG. 611. Frog skull from below. (From Wiedersheim.) For letters see fig. 612. 



sented by a pair each of ali- and orbitosphenoids in the urodeles, by 

 a ring of bone, the sphenethmoid (os en ceinture), in the anura. 

 The auditory region usually contains only prootics, the 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 

 occipital condvles. Of secondary cranial bones are to be men- 

 tioned the nasals, frontals (in many pref rentals also), and parietals, 

 the latter two fused in anura to f rontoparietals ; ventrally the large 

 parasphenoids. 



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 (pterygoquadrate) 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 (paraquadrate), and supports the lower jaw, com- 

 posed of Meckel's cartilage surrounded by membrane bones 



