114 BIRDS WITH TRUE TEETH. 



remains, and it is becoming a common thing to hear 

 of the unearthing of mastodons, elephants, etc. 



Note. — The birds of tlie present epoch are eutirely desti- 

 tute of true teeth, and the mandibles have generally more or 

 less trenchant, unarmed linear edges ; but sometimes they are 

 armed with processes of bone simulating teeth, but in no other 

 respect entitled to that name. In former epochs, however, 

 there existed types actually provided with true teeth, having 

 all the structural characteristics of those organs, and fitting in 

 sockets in the jaws. These have been combined by Marsh 

 under the general term Odontornithes (toothed birds). — Gill. 



The teeth of Hespcrornis were covered with smooth enamel, 

 terminating upward in conical pointed crowns and downward 

 in stout roots. The young tooth probably formed on the inner 

 side of the root of the tooth in use, a pit for its reception being 

 gradually made by absorption. The old tooth, being progres- 

 sively undermined, was finally expelled by its successor, the 

 number of teeth thus remaining unchanged. The teeth were 

 implanted in a common alveolar groove, as in Ichthyosaurus. 

 The skeleton measures about 6 feet from the point of the tail to 

 the end of the bill. Hesperornis regnlU appears to have had 14 

 functional teeth in the upper and 33 in the lower jaw. — Marsh. 



A fossil is the body or any known part or trace of an animal 

 or plant buried by natural causes in the earth. The molds of 

 shells, the impressions left by the feet of animals in walking, 

 implements of stone or metal, and other works of human art 

 which have been accumulated naturally into rubbish-heaps, 

 are thus strictly fossils. Perhaps the marks of rain, wind, 

 waves, and shrinkage through heat should be included. * - * 

 Fossils indicate the former existence of organic races now 

 entirely extinct ; that, as a whole, each successive period con- 

 tained more highly organized structures than its predecessor; 

 that tropical forms once flourished in the polar regions ; that 

 each epoch was characterized by peculiar groups. Hence, for- 

 mations are identified in new countries by means of fossils. — 

 a H. mtcTicod:. 



For interesting articles on Fcssil Botany, Fossil Fishes, 

 Fossil Footprints, and Fossil Forests, the reader is referred to 

 Johnson's "' New Universal Cyclopedia," vol. ii, pp. 231-0. 



