THE AMPHIBIA. 151 



But in Archegosaiirus, Pliolidog aster ^ TTrocordyliis^ Kerater- 

 peton^ Ophlderpeton, Ichthyerpeton^ the integument between 

 the thoracic plates and tlie pelvis presents regularly-disposed 

 rows of small elongated ossicles, which, for the most part, 

 converge from without, forward and inward, toward the mid- 

 dle line. No trace of these appears upon the tail, nor in 

 any part of the dorsal region of the body, nor on the limbs. 



The endoskeleton of the A.inpTiibia is least complete in 

 Archegosaurus^ where the centra of the vertebrae are repre- 

 sented only by bony rings, the ribs and the neurdl arches 

 being well ossified. In other Labyrinthodonts of the same 

 (Carboniferous) epoch, however, such as Anthracosaurus, the 

 centra of the vertebrae are completely ossified biconcave disks, 

 very like the centra of the vertebrae of Ichthyosaurus. 



In the existing JProteidea, and in the Gymnophiona^ the 

 vertebral centra are amphicoelous. In the Salamandrldea 

 they are opisthocoelous. In Pip)a and JBonibinator they are 

 also opisthocoelous, but in other Batrachia they are, for the 

 most part, procoelous, but vary in different regions, some 

 being biconvex and some biconcave. 



The first vertebra, or atlas, presents two articular cups to 

 the condyles of the skull, but there is no specially modified 

 axis vertebra. 



The transverse processes may be simple, but in the Laby- 

 rinthodonts, and in the existing Salamanders, they are divided 

 into two processes — an upper tubercular^ and a lower capitular^ 

 process. When the transverse process is thus divided, the 

 proximal end of the rib is correspondingly split into a capitu- 

 lar and a tubercular process. 



In the Gymnophiona^ the Saurohatrachia^ and the Laby- 

 rinthodonta^ the number of the vertebrae in the trunk is con- 

 siderable, and the members of the two latter groups have long 

 tails. But in the Batrachia^ the total number of vertebrae 

 does not exceed eleven, of which eight belong to the presacral 

 region, one to the sacrum, and two (modified vertebrae) to the 

 coccygeal region. The transverse processes of some of the 

 presacral vertebrae are usually very long, but there are no 

 separatel}'' ossified ribs. The transverse processes of the sacral 

 vertebra are very large and expanded, and its centrum has 

 usually a single concavity in front and a double convexity 

 behind. 



The coccyx consists of a long, cylindroidal, basal bone 

 proceeding from the ossification of the sheath of the termina- 

 tion of the notochord, and corresponding with the urostyle of 



