390 
IME. S. J. A. SALTER ON THE STRUCTURE AND 
is; blit the arrangement of the dense and softer parts is different in the two, and 
consequently the resultant form of the worn extremities is different. The central con- 
densed axis is the hardest portion of the Echinus-tooth, and this, and not its external 
border, suffers the least from wear. From this axis the worn surface passes obliquely 
downwards and outwards, and also downwards and inwards (see Plate VI. tig. 2, a, 5, d). 
It not only differs from the apex of the rodent-tooth when seen in this aspect, but, 
when viewed on the dorsal it is seen that the extremity is a point and not a cut- 
ting border ; for, from the dense central axis, the body of the tooth slopes downwards 
and outwards obliquely on either side. The apex of the worn tooth not only corre- 
sponds with the relative hardness of its parts, but is coincident with lines of superimpo- 
sition of its component elements ; and, by fracturing the tooth in its centre, where those 
elements are as yet not firmly soldered together, they sever, leaving the broken point 
similar to that produced by friction wear; indeed the worn surface seems to show that 
attrition separates and removes the imbricated elements of the tooth in layers, their 
adhesion to one another being weaker than the cohesion of their own intrinsic struc- 
ture. 
A transverse section of the entire tooth exhibits an outline like the letter T, the 
cross line constituting the body of the tooth, the single vertical line the keel. The 
dorsal surface of the body of the tooth exhibits an undulating outline, with a central 
depression, each half of the body being symmetrical : the extremities of the body slightlv 
bulge inwards at the enteric face. The keel, which springs backwards from the con- 
fluent lialves of the body in the centre of the tooth, becomes somewhat bulbous or 
dilated at its free edge (Plate VI. fig. 4, a). 
Mode of examination . — The great difficulty in successfully investigating the structure 
of the tooth of Tdchinus has arisen from the physical peculiarities of its different parts — 
the loose incoherence of its developing end in comparison with the great hardness of 
the other extremity, and the different density of the several parts of the tooth when 
fully foimecl, that is, at the same portion of the tooth in its maturity. The relation of 
the forming parts to the formed tooth can only be made out by carefully examining the 
growing end of the organ where its parts are soft, thin, and sufficiently transparent to 
be vievred microscopically in mutual relationship ; by scrutinizing the fractures of the 
tooth and the fractured surfaces in its intermediate condition of softness and hardness, 
where it is too dense to be examined microscopically as a transparent object, and too 
lax to admit of being ground down into sections, and then by comparing these results 
with the appearances presented by sections, vertical and transverse, of the mature tooth 
as seen with the microscope. 
The making of the sections of the matured parts of the tooth is another difficulty : 
the body of the organ is so dense and the keel so soft, that it requires the utmost care 
and no little dexterity, so to apply pressure and grinding action, as to render the body 
of the tooth sufficiently transparent without grinding entirely away the soft keel. The 
hard sections, when ground down sufficiently, should be mounted in Canada balsam. 
