318 



ANATOMY OF VERTEBRATES. 



to the frequent need which an aquatic animal with hot blood and 

 a quick resj^iration of air has to ascend rapidly to the surface of 

 the water. 



The cervical vertebra?, fig. 186, have short and broad centrums 

 confluent with neurapophyses ; the former developing a par- the 

 latter a di-apophysis : the pleurapophysis,7>/, is short and broad, and 

 circumscribes the ' vertebrarterial ' canal by junction with both the 

 transverse processes : which joints in the last five cervicals are 

 obliterated earlier in Ornithorliynchus than in Echidna. In the 

 latter not any of the cervicals have zygapophyses save the atlas. 

 The true centrum of this vertebra supports a great part of its neural 

 arch, and long continues distinct from that of the axis : it has a long 

 ' odontoid ' process. The lower part of the ring of the atlas sends 

 off" in Oniithorhynchus a pair of long divergent hypapophyses. 



B. Skull. — The skull in both genera of Monotremata is long 

 and low, but characterised by a relatively larger cranium in pro- 

 portion to the face than in most Marsupials. The parietes of 

 the exjDanded cerebral cavity are rounded, and their outer surface 



smooth. These characters 



202 



Base view of partially disarticulated cranial 

 segments. Echidna. Lxxix'. 



lower part of each condyle, 2, 2 : 



IS 



are most conspicuous in the 

 Echidna, in which the jaws are 

 slender, elongated, and gradu- 

 ally diminish forward to an ob- 

 tuse point, so that the whole 

 skull resembles the half of a 

 pear split lengthwise. The facial 

 angle of the Echidna is 36°, that 

 of the Ornithorliynchus 20°, 

 being almost the lowest in the 

 mammiferous class. The cra- 

 nial bones and their constituent 

 pieces continue longer distinct 

 in the Echidna than in the Or- 

 nithorliynchus, in which they 

 ultimately coalesce to a degree 

 resembling that in Birds. 



In the Echidna the basiocci- 

 pital, fig. 202, 1, is flat and 

 hexagonal, with the hind-border 

 notched to complete below the 

 laro-e vertical ^ foramen mao;- 

 num,' and contributing to the 

 these are large and formed chiefly 



