44 



t;oPK 



Fig. 12.— Ti-i7nerorhachis insignis Cope ; parts of skeleton, natural size ; a, occipital segment with 

 single cotylus ; 6, Cy posterior part of lower jaw ; d, e, series of vertebrae ; d, side view, depressed ; 

 e, view obliquely from above, both showing the rhachitomous structure : i, intercentra • p, pleuro- 

 centra. Original. From the Permian of Texas. 



The Stegocephali are only known from the Carbonic and 

 Triassic systems. They are all more or less notochordal, 

 but a considerable range in this respect is found in the Mi- 

 crosauri. In some of them the notochord is cut off by 

 the vertebral ossification, while in others (Branchio- 

 saurus) the ossification is a mere sheath round the chorda. 

 In the Ganocephali and Rhachitomi the vertebral centra 

 are represented by a segment beneath each branch of the 

 neural arch, the pleurocentrum (Fig. 12, p); and these rest 

 below on a median inferior piece, the intercentrum (Fig. 

 12, i). In the Embolomeri each of these segments is de- 

 veloped so as to form a disc, so that there are two vertebral 

 bodies to one arch. In the Microsauri the intercentra have 

 been fused with the pleurocentra, so that the body consists 



