xin PHYLUM CHORD AT A 485 



parachute or patagium in the form of lateral flaps of skin ex- 

 tending along the sides of the body between the fore and hind 

 limbs. 



Further, there are certain groups of swimming Mammals. Most 

 Mammals, without any special modification of the limbs, are able 

 to swim, and some of the quadrupeds, such as the Tapirs and 

 Hippopotami, spend a great part of their life in the water. But 

 there are certain Mammals in which the limbs are so specially 

 modified to fit them for an aquatic existence assuming the form 

 of flippers or swimming paddles that locomotion on land becomes 

 almost, if not quite, impossible. Such are the Whales and 

 Porpoises, the Dugongs and Manatees, and, in a less degree, the 

 Seals and Walruses. 



Skeleton cf Prototheria. In the Prototheria (Fig. 1058) the 

 epiphyses of the vertebrae are not well developed in the Platypus, 

 and appear to be absent in Echidna. In both genera there is the 

 normal number of vertebrae in the cervical region. The odontoid 

 process long remains separate from the centrum of the axis. The 

 cervical transverse processes are separately ossified, and only com- 

 pletely 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 thoracico-lumbar vertebrae 

 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 vertebrae. The caudal region differs con- 

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

 tail is very short, the vertebrae 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 very long, and the number of caudal vertebrae is twenty or twenty- 

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

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

 Echidna but not in Platypus there is a xiphi-sternum. The most 

 remarkable feature of the sternal apparatus in the Protheria is the 

 presence of a "["-shaped ^pi-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 Platypus, in accordance 

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



In Echidna (Fig. 1059) the squamosal extends further forwards 

 than it usually does, and the posterior root of the zygomatic arch 

 is further forward than in Mammals in general The zygoma is 



