160 Bulletin American Museum of Natural History. [Vol. XXVII) 



lacteal glands was probably a by-product of the process which transformed 

 pcecilothermal reptiles into homoeothermal mammals. The inference is 

 that Ci/nognathus possessed neither rudimentary hairs nor milk glands. 

 The large size of the pterygoids and the retention of a functional articular 

 bone indicates a corresponding development of the pterygoid muscles, and 

 this in conjunction with numerous other reptilian features in the skeleton 

 {e. g., in the femur, cervicals, pelvis) raises the presumption that reptilian 

 characters likewise predominated in the rest of the economy. Again it 

 seems likely that in Ci/nognathus the auditory ossicles were of the reptilian 

 type, since the quadrate and articular were still functional as such (p. 118). 

 Consequently there is little ground for inferring that the Cynodonts had 

 acquired external ear cartilages like those of mammals {cf. p. 125). 



While not in the least incompatible with the hypothesis that the mammal- 

 like reptiles were remotely ancestral to the Mammalia as a class, these con- 

 siderations seem unfavorable to the suggestion that they had progressed 

 very far in the direction of the Monotremes and give added weight to the 

 long list of characters (p. 156) in which the Monotremes were found to be 

 typically Mammalian. The adult skull of the Monotreme is very far ad- 

 vanced beyond any reptilian type in the side, top and palatal aspects. The 

 development of the skull is thoroughly mammalian (Gaupp) and the sup- 

 posed reptilian features are of somewhat doubtful significance (p. 155). The 

 ossicula auditus in the adult state are likewise mammalian, while in their 

 development they go as far as, but no further than, those of other mammals 

 in suggesting that the incus and malleus are transformed jaw-elements. The 

 inference is that the jNIammalia are monophyletic rather than diphyletic in 

 origin; /. c, that the common ancestors of Monotremes, Marsupials and 

 Placentals were already mammals. 



The Monotremata share with the Marsupials many important characters 

 listed above (p. 157) especially the resemblances in the auditory ossicles, 

 the possession of epipubic bones, the lack of a separate optic foramen, the 

 perforation of the sphenoid by the entocarotid artery, the characters of the 

 larynx, the fundamental characters of the Ijrain (Elliot Smith, 1894), the 

 tendency for both azygous veins to persist, etc. 



Their association with the Marsupials in the Australian fauna is also an 

 indication of common origin; and at the time when the ancestors of the 

 Marsupials had a shoulder girdle of the Monotreme type (p. 157) and were 

 also oviparous (p. 148), the gap between the two groups was evidently far less 

 than at present. 



The Monotreme stock must have begun to diverge from the Marsupio- 

 Placental remnant at an exceedingly remote epoch. Not only do they 

 retain many reptilian characters not found in higher types, notably in the 



