SKULL OF A RATTLESNAKE 



258 DENTAL SYSTEM OF MAMMALIA. 



modified tooth. This tooth is soldered to the maxillary 

 bone (Fig. 68), which rotates so as to keep the tooth laid 

 flat in the mouth at ordinary times, and to erect it when 



the deadly blow is about to be 

 Fig. 68. struck. The head of the snake 



is raised, drawn back, and the 

 fangs, erect, and exposed by the 

 Avidely open mouth, are struck, 

 by the force of the powerful 

 muscles of the head and neck, 

 \crotahis horridus). iuto the surfacc aimed at, the 



poison-bags at the same mo- 

 ment are squeezed, and their contents driven through the 

 canal in the tooth into the wound. And here may be 

 noticed the advantage of having the solid point of the 

 tootli prolonged beyond the outlet of the poison-canal, 

 and not weakened by its continuation to the apex. 



DENTAL SYSTEM OF MAMMALS. 



The class Mammalia^ like those of Bej^tiUa and Pisces, 

 includes a few genera and species that are devoid of 

 teeth; the true ant-eaters {inyrmeco2')licLga\ the scaly ant- 

 eaters or pangolins {inanis\ and the spiny monotrematous 

 ant-eater {echidna), are examples of strictly edentulous 

 mammals. The ornithorhynchus has horny teeth, and 

 the whales {halcena and halcenoptera) have transitory em- 

 bryonic calcified teeth, succeeded by whalebone substi- 

 tutes in the upper jaw. 



The female narwhal seems to be edentulous, but has 

 the germs of two tusks in the substance of the upper 

 jaw-bones; one of these becomes developed into a large 



