74 FRANK BLAIR HANSON 



reptilian sterna having a presternum in close conjunction with the 

 coracoids; a middle piece composed of the union of two longi- 

 tudinal bars, with the line of fusion clearly evident, and the 

 xiphisternal horns wide spread. A close comparison may be 

 made between this and the later stages of the mammalian ster- 

 num, except that in them all connection with coracoids is early 

 lost. 



A Chamaeleo vulgaris adult sternum (fig. 32) is the last reptile 

 considered here. It is mammalian-like and well ossified for a 

 lizard. The presternum is a large, lobate structure, bearing two 

 strong notches on either side on its posterior end, "this constric- 

 tion answering to the transverse cleft so constant in the mam- 

 malian sternum" (Parker, '68). In the mesosternum the line of 

 fusion of the two halves is well marked and extending also into 

 the presternum. The ribs articulate by synovial joints with a 

 series of enlargements on either side of the mesosternum. In de- 

 scribing the xiphisternum, Parker ('68) says, "The xiphisternum 

 has a bilobate extremity that is quite mammalian in character 

 .and no ribs ever reach this part .... the horns being 

 free from ribs, grew not only towards each other and fused, but 

 also grew backwards, so as to form a free, single xiphisternum, 

 exactly like that of an ordinary mammal. That there is no real 

 difference between these two classes in the formation of the xiphi- 

 sternum, I feel certain " 



The interesting fact about Chamaeleo is the statement of 

 Parker that behind the xiphisternum there are seven pairs of 

 floating ribs which later become fixed by growing toward each 

 other and unite by suture at the midline. This is significant in 

 the light of our contention of the non-relationship between pleural 

 ribs and sternum. There is a distinct tendency in all vertebrates 

 with ribs for these to grow ventrally. Now, if a sternum be 

 present, it is likely that they would form an articulation with it; 

 if none is present, that they should either remain free (floating) 

 or unite (true ribs) with each other in the midline. In the 

 chamaeleon both conditions are present in one animal ; those an- 

 terior thoracic ribs which, growing ventrally, met the sternum 

 and articulated with it, while those ribs immediately behind the 



