THE MOST PRIMITIVE KNOWN REPTILE. 



291 



that they are preserved on the atlas and axis in all early reptiles 

 together with a permanent primitive temnospondylous structure. 



The double-headed caudal ribs of Seymouria are unique 

 amongst Tetrapods. 



Shoulder- girdle. — I have endeavoured to show that primitively 

 the Tetrapods had only a single cartilage element on each side of 

 the shoulder-girdle, that a precoracoid was then added to this, 

 and subsequently a coracoid appeared. If this view be justified, 

 Seymouria, which has only a precoracoid and presents no trace 

 even of a cartilaginous coracoid. is primitive, more so indeed than 

 any other known lower Permian reptile. 



The preservation of all three foramina of the Ilachitomous 

 shoulder-girdle is a primitive feature known in few other 

 reptiles. 



Pelvis. — The ilium of Seymouria differs in the anteroposterior 

 elongation of its dorsal end from that of any Ilachitomous 

 Labyrinthodont, but in this character it exactly resembles the 

 four known Embolomerous ilia. It agrees with these also in the 

 process from the upper edge toward the anterior end of that 

 border. Several cotylosaur ilia resemble that of Seymouria in 

 the production of the caudal end of the upper part of the bone. 



Fore limb. — Williston has already called attention to the 

 remarkably Temnospondyl appearance of the humerus, but he 

 has also pointed out that this bone differs from all amphibians, 

 and resembles all early reptilian humeri in the presence of an 

 entepicondylar foramen. 



Hind limb. — Williston has shown how much the Seymouria 

 femur resembles that of such a Ilachitomous amphibian as 

 Eryops, and I have nothing to add to his account. 



It is, however, of great interest to note that it also presents a 

 still more striking resemblance to the Lower Carboniferous 

 (Lower Mississippian) femur which I recognised as probably 

 reptilian and called Papposaurus traquari. This bone is very 

 nearly as old as the oldest known amphibian bone, and far older 

 than any other known reptile. 



The tarsus as revealed by my skeleton is thoroughly amphibian 

 in retaining a separate intermedium. In all other reptiles 

 except Limnoscelis, where the fibulare is unossified, and the 

 Plesiosaurs and Ichthyosaurs, there are only two proximal 

 tarsals, the fibulare and a compound bone formed of the fused 

 intermedium and tibiale. 



The phalangeal formula is apparently that common to all early 

 reptiles. 



The foregoing series of comparisons show how wonderful a 

 primitive and annectant form Seymouria is. In every part of its 

 skeleton it shows a mixture of Temnospondyl and Reptilian 

 characters, each recognisable, and in general showing little 

 evidence of an intermediate condition. The whole effect of its 



