834 



PHYLUM CHORDATA : CLASS MAMMALIA 



e.co 



The vertebral centra have weak epiphyses in Ornithorhyn- 

 chus, and apparently none in Echidna. In the duckmole 

 the post-sacral vertebrae are stronger than the pre-sacral. 

 The skull is smooth and polished as in Birds, for the 

 sutures disappear. The rami of the lower jaw do not unite 

 in front, have no ascending process, and have a slightly 

 inflected angle. In Ornithorhynchus there are true mam- 

 malian teeth, but only in the young ; in Echidna none 

 are present. Cervical ribs remain distinct for a time at 

 least ; the odontoid process of the second vertebra is for 

 a long tim.e free from the centrum. Except on the atlas of 



Echidna, the cervical vertebrae 

 are without zygapophyses or 

 articular processes. The 

 (meta-) coracoids reach the 

 sternum ; there are also large 

 precoracoids (often called epi- 

 coracoids, but homologous with 

 the precoracoids of many Rep- 

 tiles and Amphibians) and a 

 T-shaped prosternum (some- 

 times called interclavicle), on 

 which the inner ends of the 

 clavicles rest, the outer ends 

 abutting on the acromion of the 

 scapulae. In Ornithorhynchus 

 the ischia form a long ventral 

 symphysis ; in Echidna the 

 acetabulum socket for the 

 femur is incompletely ossified (reminding one of Birds, 

 though it is only a secondary peculiarity) ; the pubes bear 

 epipubic bones, as in Marsupials. On the side of the 

 tarsus, in the duckmole, there is a spur perforated by the 

 duct of a gland. This spur is well developed in the males, 

 but rudimentary in the females. The male Echidna has a 

 similar but smaller spur. The fibula has a proximal 

 process like an olecranon. 



The brain is smooth in the duckmole, convoluted in 

 Echidna ; the cerebellum is not covered by the cerebrum, 

 there is a large anterior commissure, and the corpus 

 callosum is rudimentary or absent. 



Fig. 499. — Pectoral girdle 

 of Eckidna. 



sc, Scapula ; cl., clavicle ; i.cl., 

 prosternum or " interclavicle " ; 

 CO., coracoid or metacoracoid ; 

 e.co., procoracoid or precoracoid ; 

 St., mesosternum. 



