THE DENTAL TISSUES OF THE ORDER RODENTIA. 
555 
83rdj and the external about the 500th of an inch. In a longitudinal section the con- 
fluent laminae leave the dentine at an angle of 55 °, while fibres of the outer part are 
placed at an angle of 20 ° with the surface of the dentine (fig. 40). The layers have 
a thickness of from the 1500th to the 3000th of an inch. 
Myopotarnus Coy pus, Molina (Zoological Society). — The dentine of the incisors at 
its anterior termination is occupied by small rounded cells, with which the terminal 
branches of the dentinal tubes freely communicate, and in addition to which many 
areolar-shaped cavities exist similar to those found in imperfectly developed human 
and other dentine*. ) 
The enamel presents the true Hystricine character, but the layers in a longitudinal 
section are more confluent than in the teeth previously described. They leave the 
dentine at an angle of 56°, and the fibres in the outer portion of the texture lie at an 
angle of 40° with the surface of the dentine. 
In an oblique transverse section of an incisor parallel with the worn surface of the 
tooth, the fibres are seen to make four undulations, but they are less deep than those 
seen in the Porcupine and Paca. Near the terminal edge of the enamel on the sides 
of the tooth, in this as in other Hystricine incisors, the fibres make but one or two 
gentle curves. The confluent laminae, as seen in a longitudinal section, have an 
average thickness of about the 1000 th of an inch, and the enamel fibres a diameter of 
the 5000th of an inch. 
I find in the enamel of this tooth here and there irregularly rounded interspaces, 
vacant or occupied by a transparent structureless mass. This perhaps may be a 
peculiarity confined to the individual specimen. The dentine of the lower incisor 
has a thickness from back to front of the 3rd, the lamelliform portion of the enamel 
the 40th, and the external fibrous part the 500th of an inch. The cementum is con- 
tinued from the sides of the tooth a short distance upon the enamel, but does not 
blend with the terminal ends of the enamel fibres. On the contrarv, the two tissues 
are separated by a well-defined line. 
Octodon Degus, Molina (Zoological Society). — In the lower incisors the dentinal 
tubes of the anterior half of the tooth terminate in fine branches, with which few if 
any cells are intermingled. The tubes at their commencement in the pulp-cavity are 
comparatively large, and attain a diameter of about the 5000th of an inch. 
The enamel of this small tooth is truly Hystricine in character. The confluent 
layers leave the dentine at an angle of 45°, which in the outer fibrous part of the 
texture is reduced to 20 °. The 1500th of an inch is the average breadth of the layers, 
the component fibres of which have a diameter of from the 7500th to the 5000th of 
an inch. The dentine has a thickness of about the 53 th, the lamelliform portion of 
enamel the liGth, and the external part the 750th of an inch. 
In the molar teeth the enamel fibres describe a faint sigmoid curve in their passage 
* Lectures on Dental Physiology and Surgery. 
4 B 2 
