PEDIGREE OF VERTEBRATE ANIMALS. 263 



in very essential characters, which refer their supposed 

 common origin to a remote period. \Vc will mention 

 only tlie fin-like extremities of the former, wh.ich are 

 of an obviously piscine type. We are thus tlu'own 

 .])ack vaguely on such mixed forms as may have been 

 analogous to the Labyrinthodont ; nay, the ques- 

 tion arises whether the Ichthyosauria alone, or per- 

 haps the Plesiosauria with them, did not diverge 

 from the fishes independently of the other branches of 

 the reptile family; an eventuality which is taken into 

 account in the pedigree at p. 250. A certain resem- 

 blance with the skull of the tortoises (Chelonia) is 

 exhibited by that of the Dicynodonta. In them also 

 the jaws, as appears from tl:eir shape, were manifestly 

 cased in horny sheaths ; but at the same time the upper 

 jaw contained two huge tusks, and it is scarcely possible 

 to imagine a direct transition from the Dicynodonta, ap- 

 pearing in the Trias, to the more recent tortoise. In some 

 particulars of the skull, as well as in the situation of the 

 posterior nasal apertures, the forms of older crocodiles 

 exhibit an affinity with the lizards, from the older and 

 unknown forms of which they probably branched off. 

 The winged saurians, or Pterodactyles, may also be a 

 branch of the lizards. They have gained by adaptation 

 several characters, such as the shape and lightness of 

 head, the length, slenderness, and pneumatic character 

 of the tubular bones, which they share with the birds. 

 But it is not in them, but in the division comprising 

 several families which Huxley terms Ornithoscelida:, 

 cr reptiles with the legs of a bird, that we must look for 

 the actual progenitors of the birds. For among them 

 one of the most important characters of the birds is, m 



