544 
MR. TOMES ON THE STRUCTURE OF 
Jerboa ^gyptius. — The teeth of this animal present great structural peculiarities. 
The upper and lower incisors are subject to the same difference in the arrangement of 
the larnellee of the enamel as the corresponding teeth of the Dormouse ; while the 
molars resemble in structure the teeth of marsupial animals. In the long axis of a 
transverse section of an incisor, the enamel measures the 150th and the dentine from 
the 17th to the 20th of an inch. 
The dentinal tubes radiate from the pulp-cavity without exhibiting a specific pecu- 
liarity. Many of those distributed to the anterior as well as those to the lateral and 
posterior parts of the tooth, dichotomize once or twice in the early part of their 
course, and afterwards give off small branches. When near the enamel the dentinal 
tubes break up into a rich plexus of branches, many of which uniting, form a series 
of loops, and give a greater opacity to this than to any other part of the section. 
External to this plexus the dentine is comparatively transparent and traversed by a 
diminished number of dentinal tubes, a few of which are continued into the enamel ; 
these, after following the course of the lamellee, are lost in the peripheral portion of 
that tissue. The dentinal tubes at their largest parts do not exceed the 7500th of 
an inch. 
The peculiar character of the enamel of the upper incisor is best seen in a longitu- 
dinal section, in which the lameilse are shown proceeding from the dentine in straight 
lines directed obliquely upwards at an angle of 60°. In this section each layer appears 
slightly fibrous and a little indefinite in outline ; in both respects differing from the 
teeth of this and the preceding family. These conditions are partly due to the pre- 
sence of tubes continued from the dentine, and are portrayed in fig. 23. 
The layers, after proceeding for about the 250th of an inch across the enamel, sud- 
denly change their direction, and the component fibres lose the lamelliform arrange- 
ment, become parallel, and in tolerably straight lines pass upwards at an angle of 21° 
with the surface of the dentine. In this, the external part of the enamel, the tissue is 
very transparent, the fibres are rather indistinct, and the indistinctness increases the 
nearer we approach the worn extremity of the tooth. Here and there one or two 
tubes may be seen emerging from the lamelliform portion, and advancing into the 
outer transparent part of the enamel, but they are soon lost in their own minuteness. 
An oblique longitudinal section may be so made as to expose alternate layers of 
fibres cut in their length, and the intervening ones cut transversely. The latter are 
oval, and present a long diameter of about the 5550th and a short diameter of about 
the 8824th of an inch. And it will also be seen, that although the fibres have their 
long axes placed in the length of the tooth, and hence transversely to the adjoining 
fibres longitudinally exposed, yet that the long' axes are not at a right angle or any 
constant angle with them, but present various degrees of obliquity, as shown in fig. 24. 
This want of regularity in the arrangement of the fibres no doubt contributes to the 
fibrous appearance of the layers in the longitudinal section of the tootii, in which the 
component fibres are of course cut across with various degrees of obliquity. 
In a transverse section of an upper incisor the fibres of the alternate layers are 
