518 ZOOLOGY SECT. 



unite with the vertebrae at a late period, sutures being traceable 

 in all but very old animals. Zygapophyses are absent in the 

 cervical region. There are nineteen thoraco-lumbar vertebras 

 in both genera. The transverse processes are short, and the ribs 

 do not articulate with them, but only with the sides of the centra. 

 In the sacrum of Echidna there are three or four, in that of 

 Platypus two, united vertebras. The caudal region differs con- 

 siderably in its development in the two genera. In Echidna the 

 tail is very short, the vertebras depressed, with no inferior spines, 

 but with about five subvertebral bones, which differ from ordinary 

 chevron bones in being mere flat nodules. In the Platypus the tail 

 is long, and the number of caudal vertebras is twenty or twenty- 

 one ; each has a distinct inferior spinous process (infr. proc). The 

 sternum consists of a pre-sternum and three keeled sternebras : in 

 Echidna, but not in Platypus, there is a xiphisternum. The most 

 remarkable feature of the sternal apparatus in the Prototheria is 

 the presence of a T-shaped epi-sternum (interclavicle, epist.) corre- 

 sponding to that of Reptiles. The sternal ribs are ossified, and 

 are connected with the vertebral ribs by imperfectly ossified inter- 

 mediate ribs (int. rbs.*). 



The skull of the Monotremes differs widely from that of other 

 Mammals. The bones early become fused together, so that it is 

 difficult to trace their exact boundaries. The brain-case is larger 

 and more rounded in Echidna than in the Platypus, in accordance 

 with the larger size of the brain in the former genus. In both 

 genera there is a pterygoid (investing) bone not separately 

 represented in higher Mammals, corresponding to the pterygoid of 

 lower Vertebrates. The parasphenoid represents the lateral parts 

 of the parasphenoid of lower Vertebrates and the inner lamella of 

 the pterygoid process (usually regarded as the pterygoid) of higher 

 Mammals. Perforating the posterior root of the zygomatic arch 

 is a canal, comparatively wide in the full-grown Ornithorhynchus, 

 narrow in Echidna the temporal canal which is not present in 

 higher Mammals, and apparently represents the post-temporal 

 fossa of Reptiles (see p. 319). 



In Echidna (Fig. 1128) the squamosal extends further forwards, 

 and the posterior root of the zygomatic arch is more anterior 

 than in Mammals in general. The zygoma is very narrow, and 

 there is no rudiment of post-orbital processes : the jugal is 

 absent as a separate ossification. The alveolar border of the 

 maxilla (max.} is narrow and devoid of teeth. The nasal and 

 premaxillary region of the skull is drawn out into a long, narrow 

 rostrum. Near the anterior end of this is a rounded opening, the 

 external nasal opening, which is entirely bounded by the pre- 

 maxillas the nasals not extending so far forwards. An aperture 

 in the nasal septum corresponds to an actual perforation by which 

 the nasal cavities are in direct communication in the living 



