136 ELEMENTS OF VERTEBRATE PHYLOGENY 



simple; but it is sometimes difficult to know whether to attribute the 

 simplicities to primitiveness or to the secondary simplification (mistak- 

 enly called degeneracy) which is a part of their adaptation to a parasitic 

 mode of life. As regards the lamprey eye, however, there is unanimous 

 agreement among modern students that its features are all primitive 

 and show no indications of degeneracy. 



The oldest of the true fishes are the elasmobranchs, whose modern 

 representatives, the Selachii (sharks and rays) and Holocephali (chim- 

 aeras), are very different from their extinct progenitors. The elasmo- 

 branchs were derived from ancient cyclostomes, but not from lamprey- 

 like ones. Like the cyclostomes, they have cartilaginous skeletons; but 

 they also have paired fins, jaws, and scales. From those jaws have come 

 the little bones of the ossicular chain which traverses our middle-ear 

 cavity; and from some of the rows of scales on the elasmobranchs' lips 

 came their teeth, the ancestors of our own — and very different from the 

 horny teeth of lampreys. 



The primitive elasmobranchs were a main-line group, for from them 

 have come all of the higher, 'teleostome' fishes; and through these, the 

 terrestrial vertebrates. From ancient elasmobranchs there arose an ad- 

 vanced group of fishes, still with cartilaginous skeletons, called the 

 Chondrostei. These fishes have had many descendant groups, among 

 them several which might, any one of them, have given rise to land 

 forms — for they all spread into fresh waters and swamps, and developed 

 lungs of sorts, and limb-like fins with which to drag their bodies over 

 the slime. 



These lunged fishes were the Cladistia, the Crossopterygii, and an 

 offshoot of the latter called the Dipneusti — the lung-fishes proper. All 

 three of these groups were once numerous as to species and individuals, 

 but have dwindled to remnants which still cling precariously to life in 

 competition with the more advanced modern fishes. The Dipneusti, or 

 dipnoans, have but three living genera : Neoceratodus in Australia (Fig. 

 61a), the African Protopterus, and Lepidosiren in South America. 

 There are but two living cladistian genera — Polypterus and Calamoich- 

 thys, both in Africa. Until very recently it was supposed that the cros- 

 sopterygians were extinct; but one species, named Latimeria chalumnce, 

 was lately discovered in the sea off South Africa. This is the only archaic 

 teleostome known from salt water. 



The chondrosteans have persisted to the present time, but are now 

 represented only by the sturgeons (Acipenser, Huso, Scaphirbyncbus, 



