PROTOTHERIA. 683 



The vertebral centra have weak epiphyses in Orni- 

 thorhynchus, and apparently none in Echidna. In the duck- 

 mole 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 

 time free from the centrum. Except on the atlas of 

 Echidna, the cervical vertebrae are 

 without zygapophyses or articular pro- 

 cesses. The (meta-) coracoids reach 

 the praesternum ; there are also large 

 epicoracoids and a T-shaped inter- 

 clavicle, the whole girdle resembling 

 that of Lizards. An interclavicle is, 

 however, recognisable in the embryos 

 of some Placentals also. In Orni- 

 thorhynchus the ischia form a long FlG 29 s. — p e Ms of 

 ventral symphysis ; in Echidna the Echidna.— From Edin- 

 acetabulum socket for the femur is bu .''g h Museum of 

 incompletely ossified (reminding one " ence an rt - 



rl .j,i t •, . 1 j S., Sacrum; Ep., epipubic 



of birds, though it is only a secondary tones ; Ac, acetabulum ; 

 peculiarity) ; the pubes bear epipubic "/■< obturator foramen be- 



r • -Kr • 1 ^ i-i tween ischium and pubis 



bones, as in Marsupials. On the side (j>.). 



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, the cerebellum is not covered by 

 the cerebrum, there is a large anterior commissure, and the 

 corpus callosum is rudimentary, or, according to Symington, 

 absent. 



The food canal ends in a cloaca. 



The right auriculo-ventricular valve in Ornithorhynchus 

 is partly muscular as in Birds, while in other Mammals it is 

 membranous and worked by papillary muscles attached to 

 it by tendon-like cords (chordae tendineas). The temper- 



