536 
MR. TOMES ON THE STRUCTURE OF 
posterior layer.” He does not however anywhere state that the incisors of Sciu- 
ridse have any structural peculiarity resident in the enamel, by which their teeth are 
distinguished from those of other rodents ; neither am I aware that any subsequent 
author has noted its existence. Nevertheless so great a peculiarity exists throughout 
this family of Rodents, that a vertical section of an incisor, either of a Sciurus, Ptero- 
mys, Tamias or Spermophilus, may be recognized at first sight as belonging to the 
family Sciuridse. 
It has been usual to describe the enamel fibres from the view obtained in a longi- 
tudinal section of the tooth, from which circumstance the true structure has not 
been recognized. The supposed fibres are composed of layers of fibres, and each layer 
of a single series, the fibres of which are parallel to each other, and at right angles 
with those composing the layers immediately above and below. In the outer part of 
the enamel the fibres of all the layers become parallel, and the lamination ceases. 
The enamel layers, as seen in a vertical section of an incisor of Sciurus niger, and 
portrayed in fig. 6, are about the 6000th of an inch in thickness, and have straight 
and even margins. They form a right angle with the surface of the dentine, increasing 
slightly in thickness in their course outwards (fig. 6E). Each layer is composed of 
a single series of squarish fibres, laid side by side, and closely united. During the 
first part of their course they are straight and parallel, and proceed to the right in 
one layer and the left in the next, at such an angle as to produce a square pattern 
over the inner part of the enamel. The appearance thus produced in the central part 
of the enamel of a transverse section is shown in fig. 7 E. On the side of the incisor 
the decussation is less strongly marked, and at the thin terminal edges is almost lost. 
The fibres, after traversing in a diagonal course in the horizontal plane of the tooth 
two-thirds of the thickness of the enaniel, turn abruptly upwards and outwards at an 
angle of 45 degrees with their original direction. In this, the outer third of their 
course, the whole of the fibres become parallel, and in proceeding outwards make a 
gentle curve, the convexity of which is turned towards the cutting edge of the tooth. 
In the change of direction, the fibres which have followed a diagonal course make 
an angle not only in the vertical, but in the horizontal plane of the tooth, while those 
situated near tb.e terminal edge of the enamel are bent in the vertical plane only. 
The colouring matter resident in the enamel of the incisors of squirrels, is seen in 
a thin transverse section to be confined to the outer third, and looks like a stain in 
the terminal ends of the fibres, which diminishes in intensity from without inwards 
until it is lost. 
In a vertical section through the centre of an incisor, viewed by transmitted light, 
the superimposed lamince of enamel fibres will, to an inexperienced eye, appear as 
parallel fibres : a little patience will however enable the observer to see that they are 
composed of fibres cut obliquely (fig. GE). In places, faint transverse markings will 
be seen, which indicate the oblique sections of the fibres. Not unfrequently, however, 
the lateral union of the fibres is so perfect, that in the layers near the cutting edge of 
