158 Bulletin American Museum of Natural History. [Vol. XXVII, 



surus, Phascolomijs and Petrogale and in the Polyprotodonts, Perameles 

 and Dasi/icrus) the shoulder girdle passes throvigh a characteristically 

 Monotreme stage (Broom, 1899). The scapula is inclined somewhat 

 forward, the large coracoid reaches the sternum, the precoracoid is large 

 and is pierced by a precoracoid foramen, and the glenoid cavity is elongate 

 transversely and shared by the acromion. In both groups the acromion 

 and border of the spina scapulae are homologous with the curved anterior 

 border of the scapula in Cynognathus (Broom). 



(10) While the Marsupials are no longer oviparous, evidence of former 

 oviparity is seen in the large yolk sack and especially in the retention of a 

 vestigial shell membrane in the egg of Dasijurus (J. P. Hill, 1908, p. 649). 



(11) Both azygos veins persist in many Marsupials and in Echidna 

 (Beddard, 1907, p. 219). 



These and other resemblances outweigh all the differences in skull, 

 skeleton and soft anatomy and demonstrate that the ancestral lines of the 

 Marsupials and Monotremes converge into a common source which had al- 

 ready acquired many essentialh) mammalian characters. 



Naturalness of the Order Monotremata. 



The families Ornithorhynchid?e and Echidnidse, notwithstanding wide 

 adaptive divergence form a perfectly natiu'al group. Among the many 

 divergent adaptations between Echidna and Proechidna on the one hand and 

 Ornithorhi/nchus on the other are the following: 



(1) Cerebrum well furrowed in Echidna but quite smooth in Ornitho- 

 rhynchus (Weber, 1904). 



(2) Tympanic fossa (roof of tympanic cavity very small in Ornitho- 

 rhynchiis, very large in Echidna. 



(3) The differences in the shape and structure of the snout both exter- 

 nally and internally, and in the fenestra? cribrosse, or olfactory openings 

 (see p. 151). 



(4) The transitory cheek teeth of Ornifhorhynchus are multi-cuspidate, 

 while Echidna has no teeth at all. 



(5) Lower jaw extremely reduced in Echidna, but large enough to 

 crush small mollusc shells in Ornithorhynchus. 



(6) Integument more or less spiny in Echidna and Proechichia, but 

 extremely fine and fur- like in Ornithorh^jncli us. 



(7) Fore feet webbed in Ornithorhynchus, Armadillo-like in Echidna. 



(8) The difference in the astragalus (p. 154). 



(9) Dorso-lumbar vertebrae: large and powerful in Echidna, but very 

 weak in Ornithorhi/nchus. 



