ANCESTORS AND INTERRELATIONS OF VERTEBRATES 695 



Amphioxus, are the CYCLOSTOMES. These (see Chap. XV, 

 Fig. 352) are eel-like vertebrates without jaws and with a carti- 

 laginous skeleton. Next above Jthe CYCLOSTOMES come the 

 ELASMOBRANCHS (sharks, skates, etc. ; see Chap. XVI, Fig. 

 358), which also possess a cartilaginous skeleton, but are provided 

 with jaws. The direct descendants of the ELASMOBRANCHS 

 appear to be the ganoid fishes (CHONDROSTEI, CROSSOPTERYGII, 

 LEPIDOSTEI, and AMIOIDEI), which constituted the dominant 

 group during the Devonian Period (see Table XVII). Some 

 of the ganoids have a skeleton entirely of cartilage; others are 

 equipped with both cartilage and bone, but all of them possess 

 gill-covers, which are absent in CYCLOSTOMES and ELASMO- 

 BRANCHS. The bony fishes (TELEOSTS) are probably the de- 

 scendants of the bony ganoids. The lung-fishes (DIPNOI) rep- 

 resent an independent lateral branch from the ELASMOBRANCHS; 

 they are by many considered a connecting link between the fishes 

 and amphibians, but this is probably not the case. 



The AMPHIBIANS may be traced back to the ganoids and seem 

 to have developed into the STEGOCEPHALIA, a group now extinct, 

 which are the probable ancestors of not only the modern AM- 

 PHIBIA, but also of the REPTILIA. 



The most primitive living reptiles are the RHYNCHOCEPHALIA; 

 these are represented by the single living species Sphenodon 

 punctatum (Fig. 450) of New Zealand. From this group have 

 come the SQUAMATA, SERPENTES, and CROCODILINI, and some 

 of the extinct reptiles. The TESTUDINATA seem more closely 

 allied to the extinct THEROMORPHA. 



The birds have sprung from dinosaurian ancestors. They are 

 very closely related to the reptiles, and the earliest known form 

 (ARCH^OPTERYX) might almost be called a flying reptile. The 

 toothed birds are considered the forerunners of the modern 

 toothless birds. 



The MAMMALIA are of special interest, since this class of ver- 

 tebrates includes man. The earliest living mammals, the 

 MONOTREMATA, are descended from reptilian ancestors, the 



