Sir J. W. Daicson — On Dendrerpeton Acadianiim, etc. 155 



is Hyloplesion of Fritscli, and possibly not congeneric witli Hylonomus 

 proper, and certainly less reptilian in its characters. 



" The question whether Hylonomus, and especially whether Pelro- 

 bates, is to be classed with the Stegocephala or with the Ehyncho- 

 cephala, is not one which can be at once decided. The long, bent, 

 trunk ribs, the marking off of the cervical section by short differently- 

 shaped ribs, the rhombic horned episternum, the bony pubica, the 

 ossification of the carpus and tarsus taken together give both 

 quadrupeds a reptilian habitus. In the case of Petrohates this 

 becomes more pronounced by the occurrence of ventral ribs and of 

 intercentral lower arches dovetailed in between the vertebral centres 

 of the neck. On the other hand, Hylonomus and Petrohates have, 

 like the Amphibians, only one sacral vertebra. A more primitive 

 habitus is also presented in the persistence of the continuous chorda, 

 in the insignificant superficial ossification of the elements of the 

 skeleton, the absence of ossification in the heads of the hollow bones 

 of the extremities,^ and in the uniform shape of the small teeth. 



" In Hylonomus this becomes strengthened by an apparently true 

 stegocephalic skull-cover, as well as by the thick comb-like develop- 

 ment of teeth in the roof of the mouth, and the presence of a ventral 

 shield of bony scales. 



"These several points of resemblance connact Hylonomus (Hylo- 

 plesion) rather with the Stegocephala, while Petrohates, on account 

 of its ventral ribs and intercentral arches, comes nearer the RhyncJio- 

 cepliala. In other words, they both possess a general, as yet but 

 slightly differentiated, common habitus, but in Hylonomus {Hylople- 

 sion) the primitive type is more pi'onounced than in Petrohates, in 

 which already a great specialization in the direction of the Ehyncho- 

 cephala finds expression. If one, in considering Petrohates, dis- 

 regards the skull, which is not accm-ately enough known, one might 

 hold this quadruped to be a small Rhynchocephalian, of the family 

 of the Proterosauridee, if it were not that the presence of only a 

 single sacral vertebra was opposed to this view. It appears as if 

 Hylonomus, Petrohates, Palceoliatteria, and Kadaliosaurus belong to 

 a natural group of contemporaneous quadrupeds, representing how- 

 ever as many stages of specialization in the direction of Eeptiles." 



Bearing in mind that the typical species of Hylonomus, represented 

 by H. Lyelli, are in some important respects nearer to Palceohatteria 

 and Kadaliosaurus (which are regarded by all palaeontologists as 

 generalized reptilian genera tending to Amphibians) than either 

 Hyloplesion or Petrohates, this conclusion of Credner becomes very 

 significant as to the position of the Microsauria, and may be regarded 

 as confirmatory of the conclusions which, though not a specialist in 

 fossil reptilia, I have ventured to suggest in connexion with the 

 species which I have been induced to study, in consequence of their 

 connexion with my other work in Carboniferous geology. 



It must be remembered that the repositories in which Hylonomus 

 and its companions are contained are of an exceptional nature, and 

 likely to have entrapped animals specially terrestrial in their habits. 

 ^ These characters do not apply in so great degree in Hylonomus proper. 



