THE DENTAL TISSUES OF THE ORDER RODENTIA. 
557 
at an angle of 50°, which is diminished to 20° in the fibres of the outer portion of the 
tissue. In a transverse section, it is seen that the fibres in the outer part of the 
enamel are directed obliquely from the median line of the skull. The enamel fibres 
have a diameter of about the 6000th of an inch, and are intermingled in the lamelli- 
form portion of the tissue with minute rounded or oval cells, which contribute to give 
the structure a confused appearance. 
Cavia Aperea (Erxl.). — A layer of obliquely placed cells occupies the periphery of 
the enamel-coated anterior of the incisors. The dentinal tubes have slightly irregular 
parietes, and the intervening tissue has a mottled cellular appearance. In the pos- 
terior half of the tooth, the tubes as they approach the periphery throw out numerous 
characteristic thick branches, which from their number and size render a section of 
this part very opake. 
The enamel is strongly Hystricine in character, and is dotted over with minute 
rounded and branchless cells*. In a longitudinal section of a lower incisor, the con- 
fluent laminae leave the dentine at an angle of 45°, which is reduced to 20° in the 
fibres of the external part of the tissue, and the layers are about the 1500th of an inch 
thick : these characteristics are shown in fig. 44. The whole thickness of the enamel 
amounts to the 170 th of an inch, of which two-thirds is occupied by the lamelliform 
portion of the tissue. 
In the upper incisors the angle of 60° is that at which the lamellae leave the surface 
of the dentine, while the terminal extremities of the fibres lie at an angle of 20° with 
the dentine. The enamel does not exceed the 212th of an inch in thickness, of which 
five-sevenths is lamelliform. 
Cavia Kitigil (Bennett). — The dental tissues of this animal are very similar in 
structural character to those of the common Guinea Pig. The enamel is however 
less crowded with cells, and hence is much more transparent. 
Hpdrochcerus Capi/bara (Erxl.). — Many vascular canals are continued from the 
pulp-cavity into the posterior half of the dentine in the incisor teeth of this great 
rodent. A fev/ branches pass from the dentinal tubes throughout the whole of their 
course, and become more numerous near the periphery of the tissue, which in the 
anterior part of the tooth is bordered by a dense layer of ii'regular-shaped cells, that 
occupy a line the 500th of an inch thick, as shown in fig. 46. 
The enamel exhibits the true Hystricine character, both in the longitudinal and 
transverse sections, but less strongly marked than in any of the previously described 
teeth belonging to animals of this group. The laminee are more confluent, and 
the component fibres occupy a less oblique and more parallel position than we have 
been accustomed to see them ; in a longitudinal section the enamel near the dentinal 
surface is crowded with small cells, and the laminse are at this part indistinctly 
marked. A little farther out they become more strongly developed, and have the 
* The cells in the enamel of this and in all the species described in this paper, have no proper parietes, and 
should be regarded as interspaces ratber than cells in the sense now attached to that term. 
