ORIGIN OF THE LAND VERTEBRATES 103 



It is especially in the structure of the skull and 

 its relation to the jaws that the Dipnoi differ es- 

 sentially from all fishes and resemble the Amphibia 

 or lowest order of land Vertebrates. In the ordinary 

 fish the upper jaw is not fused to the skull, but is 

 loosely attached to it, the most important attachment 

 being effected by the upper portion of the first gill- 

 arch, the hyomandibular cartilage or bone, which 

 is firmly fixed to the back of the skull near the in- 

 ternal ear, and is at the same time attached to the 

 upper jaw near the articulation of the latter with 

 the lower jaw. This suspension of the jaws on 

 the skull by means of the hyomandibular is termed 

 hyostylic (Fig. 22 A), and it is characteristic of all 

 fishes except the Dipnoi. In these fishes the upper 

 jaw is completely and immovably fixed on to the 

 skull, so that, as in the land Vertebrates, the Dipnoi 

 cannot move their upper jaw without moving the 

 whole skull. The hyomandibular cartilage, which no 

 longer takes any part in the support of the jaws, is 

 reduced to a minute functionless rudiment or has 

 entirely disappeared. We thus see that the Dipnoi 

 are related to the land Vertebrates, not only by the 

 possession of similar lungs, but also in the funda- 

 mental character of their jaws and skull. The 

 spiracular cleft, i.e. the gill-cleft in front of and 

 corresponding to the hyoid arch, is, as we have 

 already mentioned, functional only in the Selachians ; 



