of the TusJcs of extinct Mastodontoid Animals. 469 



be most characteristically organized in the different groups of ani- 

 mals. The term fibres is used, moreover, to define those appearances 

 which Retzius considers due to a tubular structure, because the au- 

 thor has been unable to find anything which confirms this theoretical 

 appellation founded on the existence of a series of continuous rami- 

 fying tubes. This question therefore he leaves in abeyance. 



Mastodon giganteum. — The constituent structures of the upper 

 tusks are only two, crusta petrosa and ivory. The crusta petrosa, in 

 the specimens examined, is comparatively thin, or about half a line j 

 but the extent of the investigation being necessarily limited, the au- 

 thor considers that the observations on this head are incomplete. 



The corpuscules of the crusta petrosa are scattered irregularly ; but 

 they are numerous and give off radiating branched fibres, tending 

 generally either from the surface or to the surface of the tusk. There 

 are hardly any independent fibres. The cellular structure of the in- 

 terspaces is clearly marked. 



The junction of the ivory with the crusta petrosa is well defined by 

 a clear line, succeeded by a plumose appearance arising from a con- 

 geries of very minute ramifying fibres. This appearance looks, Mr. 

 Nasmyth says, as if it arose out of, and formed the termination of, the 

 main fibres which join the layer undivided. 



The compartments of which the main fibres are made up are par- 

 allelograms resembling those of the Elephant, and are most easily 

 observed in vertical sections, while the cellular structure of the 

 interfibral spaces is clearest in transverse sections. Minute corpus- 

 cular appearances are scattered over the substance, and so aggregated 

 as to form at intervals concentric layers. The characteristic differ- 

 ences between the structure of the tusks of the Elephant and Masto- 

 don, Mr. Nasmyth observes, consist principally in the presence of 

 transverse fibres in the crusta petrosa of the Elephant, and the greater 

 number and regularity of its corpuscules in the Mastodon, as well as 

 in the peculiar disposition to a transverse direction of its radiating 

 fibres. In the ivory the most striking peculiarity consists in the nu- 

 merous bands of corpuscular-looking bodies in its substance. These 

 appearances, so frequently observed in ivory, Mr. Nasmyth is of opi- 

 nion, depend, as pointed out by him, on the thickness of the animal 

 matter of the interfibral cells. 



Tetracaulodon Godmani. — The author says there is a great dissi- 

 milarity in the constituent structures of tusks of this Pachyderm and 

 those of the Mastodon, while on a cursory examination of the mi- 

 nute organization of these structures there is an apparent similarity. 

 The crown of both the upper and under tusk is coated with enamel 

 extending below the level of the alveolar process, with crusta pe- 

 trosa external to it, the body of the tusk being composed of ivory. 

 The alveolar process of the upper tusks is large and deep, greatly 

 exceeding that of every other tusk which the author has examined, 

 and showing, he says, that the actions in which these organs assisted, 

 must have been very powerful. 



The habits essentially necessary to the exigencies of an animal 

 being, Mr. Nasmyth observes, the same in youth as in adult age, the 



