234 TOWARDS THE HUMAN FORM 



for, apart from the Sturgeon, which goes thither only to lay 

 its eggs, it is here that the last representatives of the oldest 

 orders of Ctenobranchs are to be found : the Ganoids repre- 

 sented by the Lepidosteus and Amia in North America ; 

 the Crossopterygians localized in the rivers of Africa ; the 

 Dipnoi represented by Protopterus in Africa, Lepidosiren 

 in America, Ceratodus in Australia. These last, as we 

 have seen, are by now very well adapted for leaving 

 the fresh water and venturing on land. We have already 

 explained the mechanism by which they acquired the organs 

 that were to prepare them to live outside the water (p. 173). 

 The pioneers of the conquest of the land were very modest 

 indeed. Their skin was covered with delicate scales ; their 

 cartilaginous cranium was protected by a bony covering 

 like that of the Fishes ; between the parietals was an 

 open space which, if we may judge by the existing conditions 

 in Lampreys and certain Lizards, must have been occupied 

 by a single dorsal eye whose nerve was connected with the 

 epiphysis of the brain or pineal gland, and which has become 

 an eye for gauging temperature, a sort of a thermal eye, rather 

 than an optic one : a circle of bony pieces fixed to the sclerotic 

 surrounded the pupil. There were only four digits to all the 

 limbs : these creatures resembled Salamanders. In the 

 Carboniferous of Bohemia, Ireland, and Ohio we already 

 find Keraterpeton, whose ventral surface was covered with 

 scales and whose head bore two small horns. The European 

 species, Kereterpeton crassum, attained a length of thirty 

 centimetres, of which the tail occupied twenty. Urocordylus 

 came near it. In the Permian lakes of the district of 

 Autun the larvae of Branchiosaurus developed with external 

 brancheae, and Albert Gaudry has described them under 

 the name Protriton petrolei. We have been able to study 

 their growth from the time when the} 7 were sixteen millimetres 

 long to the adult stage, when they never exceeded sixty-four 

 millimetres. They were small Salamanders, with minute 

 scales covering their entire bodies. The vertebrae of these 

 creatures consisted only of a notochord surrounded by a bony 

 pellicle. It was the same with the Dolichosomaatidae, which, 

 although they preserved their external gills, had already lost 

 their limbs and elongated their body until they had one 

 hundred and fifty vertebrae to the length of one metre. 



