1264 



CLASS MAMMALIA. 



that of Reptiles in that the acromion is situated on the anterior or pre- 

 axial border, which in the higher Mammals has become twisted round 

 to the dorsal surface to form the spine of the scapula. The scapula of 

 some of the Anomodont Reptiles appears to indicate how this torsion of 

 the preaxial axial to form a ridge on the dorsal surface has taken place. 



At the close of the preceding chapter reference has been made to 

 the relationship of the Prototheria, or rather of its existing repre- 

 sentatives the Monotremata, to Reptiles and Amphibia. The struc- 

 ture of the pectoral and pelvic girdles presents, indeed, so many 

 striking resemblances to the same parts of the skeleton in the 

 Labyrinthodont Amphibia and Anomodont Reptiles, as to indicate, 

 as already mentioned, a distinct genetic connection between the 

 three groups ; the characters of the humerus apparently indicating 



Fig. 1130. — A, Lateral view of skull of Echidna; b, Do. of Ornithorhynchus ; c, Oral view of 

 mandible of do. ; d, Sternal region of do. c, Clavicle; i, Interclavicle ; p, Presternum; ms, Meso- 

 sternum ; r, Ribs ; sr, Sternal ribs ; ir, Intermediate ribs ; m, Cornules. Reduced, (d after 

 Flower ; the others after Giebel.) 



that the nearest relationship is with the Anomodonts. The de- 

 ciduous teeth of Ornithorhynchus indicate, moreover, without any 

 doubt that the ancestors of the Monotremes were provided with 

 persistent teeth, which were probably monophyodont. More- 

 over, a distant resemblance between these deciduous teeth and 

 the cheek-teeth of the extinct group, mentioned below under the 

 name of Multituberculata, suggests that these forms may themselves 

 be Prototheria. If this should prove to be the case, it would then 

 be apparent that that branch of the subclass could not have been 

 the ancestral stock of the Marsupials ; and we shall accordingly 

 have to look for another group or order of Prototheria, with a 

 dentition akin to that of the Polyprotodont Marsupials. It is, 



