72 FRANK BLAIR HANSON 



horns. No ribs are attached to these horns, but three pairs reach 

 the anterior portion, to which they are but feebly attached. If it 

 be assumed that the presternum here is of necessity derived either 

 from the coracoids or from the ribs, the answer can only be that 

 it musl have come from the former. This sternum with its 

 great horns reappears in Manis longicauda (Parker, '68). 



A scries of three figures (figs. 27, 28, and 29) gives an idea of 

 how the mesosternum and xiphisternum are formed. In figure 

 27 the presternum is as before, and the two bars extend caudally. 

 This is very similar to the amphibian Calamites (fig. 24), and the 

 suggested series of changes outlined in the description of Cala- 

 mites necessary to make of it a typical reptilian or mammalian 

 sternum are progressively illustrated in these three reptiles. 



In figures 28 and 29 the xiphisternal bars, by a fusion along 

 their medial surfaces, have formed a middle sternal piece or 

 mesosternum. The posterior ends of the coalesced bars remain 

 apart in the xiphisternum. 



In the mesosternum is a sternal fossa, where the union was not 

 complete. This may persist throughout life in many forms 

 (Varanus, Crocodilia) or, as in others, close up later, leaving a 

 whitish streak to indicate the line of fusion. This fontanelle is 

 also common in mammals, but there it is usually located in the 

 xiphisternum, and I have also repeatedly observed the whitish 

 streak of hyaline cartilage in the mesosternum in fetuses of pigs 

 and mice. It would seem from this reptilian material, and avian 

 and mammalian material agree, that the presternum is a product 

 of the coracoids, and this in turn gives off two backward prolon- 

 gations, which, fusing throughout a greater or lesser part of their 

 extent, form the mesosternum and xiphisternum. It is hardly 

 necessary to again point out the feeble relation of ribs and ster- 

 num in these last three figures. 



Chirotes (fig. 30) gives the completion of the series; it is a 

 sternum of the utmost importance in the consideration of the 

 problem. Parker's description is so trenchant that a part is 

 quoted : 



In the whole range of vertebrate morphology there is nothing more 

 beautiful or more instructive than the relatively large sternum of Chi- 



