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 question 

 arises whether the Ichthyosauria alone, or perhaps 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 pedi- 

 gree at p. 250. A certain resemblance 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, appearing 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 farnilies which Hux- 

 ley terms Ornithoscelidae, 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 im- 

 portant characters of the birds is, in some genera, in 



