136 THE VERTEBRATE SKELETON. 



and is very little broken up by fossae. Paired dermal supra- 

 occipitals are found, and there is an interparietal foramen. 

 The epi-otics and opisthotics form a pair of bones distinct from 

 the exoccipitals. Four simple limbs of moderate length are 

 generally present, and in some cases all four limbs are pente- 

 dactylate. Among the better known genera of Labyrinthodonts 

 are Mastodonsaurus, Nyrania, and Archegosaurus. 



Order 3. GYMNOPHIONA J . 



These animals form a group of abnormal worm-like Am- 

 phibia having an exoskeleton in the form of subcutaneous 

 scales arranged in rings. The vertebrae are biconcave and are 

 very numerous; very few however belong to the tail. The 

 skull has a complete secondary bony roof, the mandible 

 bears teeth and has an enormous backward projection of the 

 angular. The hyoid arch has very slender cornua and no 

 distinct body, it is attached neither to the cranium nor to 

 the suspensorium. The ribs are very long and there are no 

 limbs or limb girdles, 



Order 4. ANURA. 



These are tailless Amphibia, which except in a few in- 

 stances, are devoid of an exoskeleton. The vertebrae are as 

 a rule procoelous, and are very few in number. The post- 

 sacral part of the spinal column ossifies continuously, forming 

 an unsegmented cylindrical rod, the urostyle. Remains of the 

 notochord persist, lying vertebrally, i.e. enclosed within the 

 centra of the several vertebrae, and not as in Urodela lying 

 between one vertebra and the next. The skull is very short 

 and wide. The mandible is almost always, if not invariably, 

 toothless. 



The f rentals and parietals on each side are united so as to 

 form a pair of fronto-parietals, and a girdle-like sphenethmoid 

 is present. 



1 See R. Wiedersheim, Anatomie der Gymnophionen, Jena, 1879. 



