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semi-transparent, and without radiating fibres in the external half; 
but those situated in the internal half are of the usual opacity, and 
give off numerous radiating fibres. Transverse, irregularly beaded, 
independent fibres traverse the substance, making one distinct curve 
in their passage across it. The cells of the interspaces are slightly 
visible. 
The ivory is so translucent and homogeneous as to exhibit generally 
very little character. The fibres undulate but do not divide, forming 
an abrupt line of junction with the crusta petrosa. The form of the 
beaded compartments of the fibre is oblong, not rounded, as in 7. 
Kochu, and they do not exhibit thorny projections. These are the 
only marked differences in the two species. 
The cells of the semi-transparent interfibral space are generally 
visible. 
Missourium.—The constituents of the tusks are likewise crusta pe- 
trosa and ivory; but their intimate structure, Mr. Nasmyth says, is 
more peculiar, so far as his examination has extended, than that of 
the tusks of the preceding animals. 
The crusta petrosa, in the section which the author was permitted 
to make, was more than three-eighths of an inch thick. The corpus- 
cules were very numerous, and generally within the distance of one 
diameter. The granulated compartments of which the corpuscules 
were composed, were very visible, and often without radiating fibres, 
but where these occurred they were of a coarse structure. The 
transverse independent fibres were beaded in coarse, somewhat tor- 
tuous, ovoid compartments, and ranged very close to one another, 
with interfibral spaces of about only two fibral diameters, and followed 
a straight, perpendicular and parallel course to the surface. The 
cells of the semi-transparent interfibral space were generally visible. 
The appearances presented by the ivory at its junction with the 
crusta petrosa, Mr. Nasmyth was unable to ascertain; but in the 
substance of the ivory the fibres undulated, and their beaded com- 
partments had a rounded shape: these fibres were frequently in- 
vested with an irregular congeries of granules distinct from the inter- 
fibral cells. Towards the central portion of the ivory the compart- 
ments forming the fibre were frequently so disposed as to give the 
fibre a peculiar tortuous appearance. 
The peculiarities of the tusk of the Missourium are given by Mr. 
Nasmyth as follows ; and, he says, they would certainly indicate a di- 
stinct species of Mastodontoid animal :— 
1. The great extent of the crusta petrosa. 2. The close aggre- 
gation of its corpuscules. 3. The granulated structure of these cor- 
puscules. 4. The coarse granulated structure of the compartments 
of the radiating fibres. 5. The close parallel perpendicular arrange- 
ment of the fibres of the crusta petrosa. 6. The irregular congeries 
of granules surrounding the fibres of the ivory. 7. The peculiar tor- 
tuous appearance occasionally exhibited by these fibres. 
On the whole, Mr. Nasmyth observes, the several species of ani- 
mals noticed in his paper seem to be nearly allied, and fitted to 
exist under nearly similar conditions; and though the early eras to 
