PEDIGREE OF VERTEBRATE ANIMALS. 263 
in very essential characters, which refer their supposed 
common origin to a remote period.- We will mention 
only the fin-like extremities of the former, which are 
of an obviously piscine type. We are thus thrown 
back 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 their 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 Ornithoscelide, 
or 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, in 
