552 
MR. TOMES ON THE STRUCTURE OF 
gins, and run principally in the transverse axis of the tooth. The tissue of the 
cementuin, as seen in a transverse section, is unusually clear and transparent. 
I am indebted to Mr. Waterhouse for a list of those rodents which he considers 
the most typical species of the several divisions of his family Hystricidie, and have 
been fortunate in obtaining the teeth of all he has enumerated, excepting Echimys. 
In the dentine of these teeth, I find nothing that is characteristic of the group. 
But the enamel is very peculiar, and the peculiarity is constant in' each species I 
have examined. So strongly is the family characteristic marked by the peculiar ar- 
rangement of the fibres of this tissue, that it is necessary to have seen the structure 
in one hystricine tooth only, to be enabled to recognize at first sight the tooth of any 
other species as a member of the same group. We no longer see in a longitudinal 
section of an incisor uniform laminae separated in the lamelliform portion of the 
tissue by well-defined lines, but in their stead find thick and confluent layers of ob- 
liquely placed fibres, which in a transverse section are seen to pursue a serpentine 
course from the dentine towards the surface, near to which they become straight and 
parallel as in the corresponding part of the enamel of the teeth previously described. 
Hence in this as in the preceding group, I need describe minutely the structure of 
the incisors of one species only. The molar teeth offer greater variety in the arrange- 
ment of the component tissues, and may require farther notice. 
Hystrix cristata (Linn.). — In the incisors, the enamel attains a thickness of about 
the 45th and the dentine the 4th of an inch. The latter tissue offers no striking pe- 
culiarities ; near the enameled surface it is tenanted by obliquely placed elongated 
cells, similar to those found in the incisors of the Beaver and many other teeth : they 
are shown in fig. 35 D. 
In a longitudinal section, the enamel presents a very beautiful and, as compared 
with the tissue as it occurs in the preceding families, a very novel appearance. Large 
confluent laminae of from the 1000th to the 1500th of an inch in thickness, leave the 
dentine at an angle of 80°. Each layer is composed of enamel fibres directed obliquely, 
and the obliquity varies in the adjoining layers, but corresponds in the alternate ones, 
in the manner delineated in fig. 35 E. But the fibres are not oblique in one direction 
only ; one extremity of each fibre may be traced to dip into or under those of the 
contiguous layers, while the other extremity is usually cut obliquely across, and 
exhibits a diameter of about the 5000th of an inch. When within the 250th of an 
inch of the surface the layers gradually disappear, and their component fibres take 
a parallel course, and at an angle of 30° proceed to the surface. 
In an oblique transverse section parallel with the course of the enamel laminae, 
the appearances are even more striking than in the longitudinal one. The enamel 
looks as though the fibres were thrown into waves, the furrows of which commence 
at the surface of the dentine, and proceeding obliquely outwards, crop out where the 
fibres become parallel, and form the external portion of this tissue ; their direction is 
shown in fig. 36. A close inspection of a favourable section will enable the observer 
