ECOLOGY AND BIOLOGY OF THE PACIFIC WALRUS 



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The clinical crown is the entire distal part of the tooth that is exposed beyond the 

 gingiva. It may comprise not only the anatomical crown but part of the 

 anatomical root, as well. The clinical root is the remainder of the tooth, 

 proximal to the gingiva. 



Eruption is the gradual emergence of a tooth out of its alveolus or socket, until 

 the anatomical crown is fully exposed. Alveolar eruption is the emergence of the 

 crown above the rim of the bony socket. Gingival eruption is emergence of the 

 crown above the gingiva. Distad migration is the further movement of the tooth, 

 after the crown is fully erupted. When a tooth is gingivally erupted to the extent 

 that it touches any of the teeth in the opposing tooth row, it is said to be in 

 occlusion with its antagonist. When the mouth is closed to the extent that the 

 fully erupted teeth meet, the dentition is said to be occluded. 



Each of the developing primary and secondary tooth germs lies in a spacious 

 follicle of soft tissue, which is entirely or partly within an open pocket in the 

 bone. This pocket is the alveolus. The term "crypt" is not appropriate in the 

 walrus, since no follicles are completely enclosed by bone. Later, when the 

 development of the tooth is well advanced, the root is enclosed in a tightly fitting 

 bony socket, to which it is attached by fibers of the periodontal membrane. 



The proximal end of the root is the root apex, in which is an aperture, the 

 apical foramen. Through the apical foramen pass the neural and vascular con- 

 nections that serve the dental pulp. The pulp is a soft, gelatinous tissue situated 

 within the pulp chamber of the clinical root. Its peripheral cells (odontoblasts) 

 produce the dentin which makes up most of the body of the tooth. Over the outer 

 surface of the clinical root, cementum is deposited by cementoblasts of the perio- 

 dontal membrane. The production of calcified dental tissues by odontoblasts and 

 cementoblasts is known as apposition. It comprises the formation of successive 

 layers of organic matrix and deposition therein of calcium salts. Growth of the 

 tooth is therefore by accretion, i.e., it is cumulative from addition of increments 

 laid down upon those already present. This is in contrast to the intussusceptive 

 growth of most bones and other tissues, wherein new material is formed among 

 that already present. The calcified dental tissues are permanent, to the extent 

 that their structure and composition are not modified during further 

 development of the tooth. However, they can be removed (decrement) by several 

 processes: attrition is a wearing away at the locus of contact with opposing teeth; 

 abrasion is by frictional contact with materials other than the opposing teeth; 

 fracture occurs especially in the tusks from percussive contact with hard objects 

 or excessive torque; resorption is the chemical removal of tissue by osteoclasts of 

 the periodontal membrane. I have seen no evidence of dental caries (cavities) in 

 walrus teeth. 



Ontogeny of Prijnary and Secondary Dentitions 



The teeth of Odobenus are in a dynamic state of evolutionary change, as is 

 evident from their extreme irregularity in occurrence, size, and position. This 

 variation has led to marked controversy among naturalists as to dental 

 homologies and the proper descriptive notations for the walrus' dental formulae. 

 I investigated the ontogeny of both the primary and the secondary dentitions in 

 some detail in a series of small fetuses provided by A. W. Mansfield and J. J. 



