777 
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, sueceeded 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 soageregated 
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 
organization of the individual tissues is the same at both periods, 
though certain modifications of instruments are exacted at successive 
stages of existence. Thus, in early youth, when the frame is not 
powerful, every efficiency is given to the cutting edges of the dental 
apparatus ; and the author states a fact he believes never before re 
marked, though long noticed by himself, that the tusks of the young 
Elephant and Walrus are tipped with a very thin layer of enamel. © 
The head of the Tetracaulodon Godmani examined by Mr. Nas- 
myth is shown to have been that of an animal in which two of the 
adolescent teeth are well developed. The crusta petrosa of the tusk 
was about half a line thick, and extended over the whole of the visible 
surface. The corpuscules were irregularly disposed, but closely ag- 
ervegated, and exhibited in the transverse section an irregularly circu- 
