556 
MR. TOMES ON THE STRUCTURE OF 
outwards, where the tissue exists as a thin layer ; but in those parts in which it is 
more abundant in quantity, the fibres in the external pait of their course assume the 
confluent lamelliform arrangement. 
Schizodon fuscus (Waterh.). — In the incisor teeth, a few vascular canals are con- 
tinued from the pulp-cavity into the posterior half of the dentine. The dentinal tubes 
branch from their commencement and terminate in the anterior part of the tooth in 
fine tubules without the presence of peripheral cells. They have a diameter of about 
the 6000th of an inch. 
The enamel has a general resemblance to that described in the last species. The 
confluent laminae have a thickness of about the 2300th of an inch and leave the 
dentine at an angle of 50°, which in the fibres of the exterior is reduced to 15°. The 
dentine has a thickness of about the 15th, and the enamel the 166th of an inch, of 
which 13 parts are lamelliform and 5 parts fibrous and external. 
In the molar teeth the enamel fibres advance one third of their course in parallel 
lines, and then fall into the confluent lamelliform arrangement, but with less precision 
than in the incisor teeth. 
Spalacopus Pcsppigii (Wagner). — The dental tissues of the incisor teeth of this 
animal resemble pretty closely those of the preceding species. The posterior half is 
not however permeated by vascular canals. The enamel has a thickness of about 
the 150th of an inch, of which seven-tenths is lamelliform, and three-tenths external 
and fibrous. The inner part of the external division is tenanted by small rounded 
cells, wdiich confuse the fibrous character of the part. The laminae lie at an angle of 
50° with the surface of the dentine, while the fibres of the outer part pass upwards 
at an angle of 25°, and finally turn outwards and advance to the surface at an angle 
of 90°. 
Hahrocoma Bemiettli (Waterh.). — In the incisors of this creature, a few vascular 
canals are continued from the pulp-cavity for a short distance into the posterior half 
of the dentine. The dentinal tubes give off short branches from their commencement, 
undulate irregularly, and are not very uniform in size. They end without the presence 
of an anterior peripheral layer of cells. The confluent layers of enamel fibres leave 
the dentine at an angle of 50°, and have a thickness of about the 1 666th of an inch. 
The fibres of the outer portion of this texture lie at an angle of 15°, the thickness of 
this part being about i\ths, while the lamelliform portion occupies yfths of the 
whole thickness, which measures about the 108th of an inch in thickness: fig. 42 
and 43 show the texture in a longitudinal and a transverse section. The molar teeth 
of this species of Hahrocoma closely resemble those of Schizodon fiiscus. 
Chinchilla lanigera (Molina). — The dental tissues both of the molar and incisor 
teeth strongly resemble those of the four preceding species. In the incisor the dentine 
is without a peripheral layer of cells in the anterior half, and without vascular canals 
in the posterior half of the tooth. In a longitudinal section the confluent layers of 
enamel fibres have a thickness of about the 1500th of an inch, and leave the dentine 
