420 THE VERTEBRATE SKELETON. 



by a similar plate, the tail is generally encased in an unjointed 

 bony tube, and there is commonly a ventral plastron. 



In Phocaena phocaenoides the occurrence of vestigial dermal 

 ossicles has been described, and in Zeuglodon the back was 

 probably protected by dermal plates. 



TEETH 1 . 



Teeth are well developed in the vast majority of mammalia, 

 and are of the greatest morphological and systematic import- 

 ance, many extinct forms being known only by their teeth. 

 Mammalian teeth differ from those of lower animals in various 

 well-marked respects. (1) They are attached only to the 

 maxillae, premaxillae and mandible, never to the palatines, 

 pterygoids or other bones. (2) They frequently have more 

 than one root. (3) They are always, except in some Odon- 

 toceti, placed in distinct sockets. (4) They are hardly ever 

 ankylosed to the bone. (5) They are in most cases markedly 

 heterodont. (6) They are commonly developed in two sets, 

 the milk dentition and permanent dentition. 



It sometimes happens that teeth after being formed are 

 reabsorbed without ever cutting the gum. This is the case, 

 for instance, with the upper incisors of Ruminants. 



The form of mammalian teeth varies much, some are simple 

 conical structures comparable to those of most reptiles, and 

 these may either have persistent pulps, as in the case of the 

 upper canines of the Walrus and the tusks of Elephants, or 

 may be rooted as in most canine teeth. Some teeth have chisel- 

 shaped edges, and this may be their original form, as in the 



1 See W. H. Flower, "Remarks on the homologies and notation of the 

 teeth in Mammalia," J. Anat. and Physiol. norm, path., Vol. iii., p. 262; 

 R. Owen, Odontography, London, 1840 45; C. S. Tomes, Manual of 

 Dental Anatomy, London, 1876. See also H. F. Osborn, "Recent re- 

 searches on succession of teeth in Mammals," Amer. Natural., xxvu., 

 p. 493, and "Rise of Mammalia in N. America," Stud. Biol. Lab. Columb. 

 Coll, Zool. i., no. 2. 



