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It is probably the oldest true ossified sternum known. Watson believes he 
has discovered a sternum in ” Seymouria ” — “the first evidence of an 
ossification in this element in a Lower Permian Reptile.” For various 
reasons I differ from Watson as regards the affinities of Seymouria” and 
believe it to be an Embolomerous Amphibian. The ribs are quite unlike 
the ribs of any reptile, and it seems to me more probable that the element 
discovered by Watson is the homologue of the Xiphisternum of the frog 
and not a true costosternum. If this be so the stermim of Youngina would 
be the earliest known true sternum. 
The tarsus and metatarsus are exceptionally interesting. Only a few 
years ago Goodrich called attention to the importance of the tarsal and 
metatarsal condition as throwing much light on reptilian classification. He 
divided the later reptiles into two branches — those with simple metatarsals 
and those with a specialised fifth metatarsal. It had long been known that 
Sphenodon lizards and Chelonians have a peculiarly hooked fifth meta- 
tarsal. In Goodrich’s opinion such a peculiarity is not likely to have been 
twice evolved, and if that be so those forms with this specialisation must 
be closely related. The discovery of a two arched reptile with a simple fifth 
metatarsal must mean, if Goodrich’s opinion be correct, that the Chelo- 
nians are derived from a two arched reptile like Sphenodon with remarkable 
specialisations of the skull. The other alternative is that the Chelonian fifth 
metatarsal has been independently evolved and come to resemble that of 
Sphenodon by a parallel development. Personally, I favour the latter 
alternative. 
