238 TOWARDS THE HUMAN FORM 



The Microsaurians were Stereospondyles ; they had dermal 

 scales, dorsal as well as ventral, arranged on the belly like 

 those of Stegocephala ; their pelvis, largely cartilaginous, 

 had only two osseous discs widely separated from each other. 

 They have also been classed among the Batrachian 

 Stegocephala, but they have feet with five digits and mobile 

 chevron-bones on their caudal vertebrae, and iliac bones which 

 are articulated to two vertebrae instead of one. These 

 characteristics are common to the Reptiles, and in the absence 

 of embryogenetic data we can only make purely conventional 

 distinctions between these primitive groups. The Hylonomes of 

 the Carboniferous of Nova Scotia, their near relative Hyloplesion 

 of the Upper Permian of Bohemia, which was only a decimetre 

 in length, Seeleyia, only four centimetres long, and Melaner- 

 ■peton and Orthocosta were all kindred forms. In Petrobates the 

 ventral armour showed a striking resemblance to the abdominal 

 ribs which we shall find in the Rhynchocephala, and which 

 also exist in Crocodiles. 



The Rhynchocephala, sprung from the Microsaurians, are 

 probably the stock whence all the other Reptiles diverged. 

 Their biconcave, stereospondylous vertebrae are separated by 

 spaces, and bear caudal chevron-bones ; the quadrate bone is 

 fixed ; they have abdominal ribs formed of disjointed parts 

 arranged in chevrons as though, by a process analogous to that 

 already encountered in fishes, the ventral dermal bones were 

 only embedded in the wall of the abdomen, and did not show 

 any superficial dermal ossification. Their teeth are planted in 

 the sharp edge of the jaws and have no alveoli. A pelvis 

 similar to that of the Microsaurians still persists in Palczohatteria 

 of the Permian sandstones of Saxony, and the Protorosaurians 

 of the magnesian limestone of Thuringia. These were lizards 

 of about one and a half metres long, and they lead up to the 

 true Rhynchocephala with their completely ossified pelvis ; 

 among which are to be included Callibrachion, reconstructed 

 by Boule and Glangeaud, and Sauravus costei, described 



anterior dorsal parts, the basi-dorsals ; two ventral anterior parts, the basi- 

 ventrals ; two dorsal posterior parts, the inter-dorsals ; two ventral posterior 

 parts, the inter -ventrals. Those animals are by general agreement regarded as 

 Batrachians in which these parts have remained distinct, at least in the caudal 

 region, and those in which the inter-ventrals are missing, since the half of 

 each vertebra is simply tripartite. Those forms in which the half-vertebras 

 are likewise tripartite and the inter-dorsal is missing are classed as Reptiles. 

 This is the case with Eryops and Cricotus. 



