Mammalia: Visceral Systems (,1)', 



are situated only on the jaws, whereas in fishes there may be numerous 

 teeth on the roof of the mouth and even on the hyoid and branchial 

 arches. 



The number of teeth varies within broad limits. The following 

 statements concerning number refer to functional teeth in the adult. 

 Quite toothless are the Australian duckbill (Ornithorhynchus) and 

 spiny anteater (Echidna) ; South American anteaters (Myrmecophagi- 

 dae) and the Asiatic and African pangolins (Manidae) ; whalebone 

 whales (Mysficeti) and the female narwhal (Monodon). The ex- 

 tremes in number of teeth occur in the one Order, Cetacea. The 

 male narwhal (Monodon) has normally only one functional tooth 

 which is exaggerated into a slender, straight tusk protruding directly 

 forward from the left side of the upper jaw (Fig. 482). The correspond- 

 ing right tooth is rudimentary. In females both are rudimentary. 

 Spiral grooves on the surface of the tusk indicate a twist acquired dur- 

 ing its growth. Its length is one-third to one-half that of the body, or as 

 much as 9 or 10 feet in full-grown narwhals. Certain beaked whales, 

 Mesoplodon and Ziphius, and the "bottlenose" whale, Hyperoodon, 

 have only one pair of functional teeth in the lower jaw and none in the 

 upper. In various other cetaceans the number ranges upwards — several 

 or many pairs, and situated on both upper and lower jaws, but only on 

 the lower jaw of sperm whales — to a maximum of 200 or more in some 

 dolphins. In heterodont placental mammals a typical permanent 

 dentition includes 44 or 48 teeth, but in many mammals the number 

 is less than that and in some it is greater. 



Replacement of a first or "milk" dentition by a second or perma- 

 nent dentition (the diphyodont condition) may fairly be said to be 

 characteristic of mammals, in contrast to the indefinitely repeated 

 replacement (polyphyodont condition) of other vertebrates. The 

 second dentition endures throughout the animal's life or, in event of 

 loss of any of its members, they are not replaced. There are, however, 

 some mammals which have only one obvious dentition, and these have 

 been described as monophyodont (marsupials, some insectivores, 

 cetaceans, sea cows). But the embryos of "monophyodont" mammals 



Fig. 482. Narwhal skull. Bone has been removed to expose the root of th( 

 tusk and the rudimentary tusk of the other side. (Courtesy, Flower and Lydekker 

 "Introduction to the Study of Mammals," London. A. & C. Black, Ltd.) 



