THE DENTAL TISSUES OF THE ORDER RODENTIA. 
543 
Myoxus avellanarius (Linn). — In the upper incisors of the Dormouse may be ob- 
served the first indications of the peculiar arrangement of the enamel layers, which 
holds in the great majority of the members of the family Muridse. 
The arrangement of the component layers of the enamel is, however, not alike in 
the incisors of the upper and lower jaws ; hence a description of each is needed. 
In the upper incisors, the enamel is composed of fiexuous layers of fibres arranged 
transversely to the long axis of the tooth. In a longitudinal section each layer is 
seen to proceed upwards, then obliquely outwards, and afterwards again upwards, the 
general direction being at an angle of 70° with the surface of the dentine. The layers 
have serrated margins through the first curve, but during the after part of their 
course the margins are even, as in the Sciuridm. The layers, after extending the 
500th of an inch across the enamel, are broken up and the fibres are continued in 
parallel lines through the 750th of an inch to the surface, and at an angle of 30° with 
the surface of the dentine : these characters are shown in fig. 18 E. 
The enamel layers are subject to a little variety in thickness, the average being 
about the 7500th of an inch ; and here and there a layer may be found which gradually 
diminishes till it comes to a point and is lost before reaching the external portion of 
the enamel. An oblique section in the length of the tooth may be made at such an 
angle as to expose one layer of fibres in their length and the adjoining layer cut 
transversely. This view is shown in fig. 19. It will be seen that the fibres have a 
greater breadth than thickness. 
In a transverse section the component fibres of one layer are shown crossing those 
of the layer above and below at a right angle, thus producing a square pattern over 
the lamelliform portion of the enamel, which is represented in fig. 20. The fibres give 
a transverse measurement of the 8823rd of an inch. The fibres in the outer division 
of the enamel are straight, and lean obliquely towards the median line of the skull. 
In the lower incisors of this little creature the position of the enamel layers is re- 
versed. A transverse section exhibits them extended in the length of the tooth, pre- 
senting an appearance similar to that seen in a longitudinal section of the corre- 
sponding upper tooth, excepting that they make but one curve with a flattened and 
slightly enlarged middle portion (fig. 22). A longitudinal section of a lower incisor 
presents an appearance similar to a transverse section of the corresponding upper 
tooth, as shown in fig. 21. 
The dentine of the incisors of the Dormouse is not sufliciently peculiar to render a 
minute description necessary. The tubes are perceptibly larger at the lower than at 
the upper part of the tooth, and those in the posterior half of the tooth retain their 
full dimensions till near their termination. 
The enamel is about the 300th, and the dentine the 23rd of an inch thick. 
In the molar teeth, the lamelliform arrangement of the enamel exists in a percep- 
tible degree near the terminal edge only, and so far establishes a resemblance to the 
molars of the Rat-tribe. The dentine offers no peculiarity worthy of notice in this 
communication. 
