GEOWTH OP THE TOOTH OP ECHHSHS. 
401 
ground into section, exhibited in the illustration, Plate VII. fig. 5, 5, the individuality 
of the fibres is displayed in their lateral narrowness, and their crystalline fracture 
is also seen. This latter circumstance is more distinctly shown in a vertical section of 
a much larger tooth (also imperfectly consolidated), magnified twice as many diameters 
(400), at Plate VIII. fig. 8, h ; and the rhombic character of the fracture is here clearly 
traceable. 
The fibres, like the plates, ultimately become united together by the soldering par- 
ticles, but their adhesion is never so complete as that of the plates. It will be seen in 
the sections figured in Plate VII. fig. 5, that though they assume a more or less poly- 
hedral form for the sake of closer packing, the angles are never sharp and pointed. 
This has already been alluded to as explaining the position and the linear arrangement 
of the lacunse, as seen in vertical section. 
The keel fibres are free at their distal extremities until they become consolidated by 
the soldering particles ; at theii- proximal ends they are from the first united inter se : 
numbers of them may be detached together from the edge of the plate upon which they 
hang without disturbing them relation among themselves. 
In large teeth the keel fibres sometimes attain the i^th or i^th of an inch in length : 
one, the of an inch, is represented at h, fig. 7, Plate VI. 
The description I have given applies to keel fibres in the teeth of the small variety of 
E. sphcera, upon which my researches were made : in E. Flemingii I have found that the 
fibres slowly and evenly enlarge in proceeding from the proximal to the distal extremity, 
instead of swelling out club-shaped, towards their free ends. 
Flahelliform Processes. — x'Uso attached to the upper edge of the primary plates, and 
usually commencing to be formed before the appearance either of the secondary plates 
or the keel rods, there is developed towards the superior enteric angles a sprouting of 
moss-like reticulations (Plate VI. fig. 6, and Plate VII. fig. I). These reticulations 
gradually but not rapidly enlarge, spreading on either side in a dhection downwards and 
outwards. They do not stand away at an angle from the plates, like the keel rods, but 
pass sidewise between them, thus separating each plate from the next in previous con- 
tact with it. It is this elaborate reticulation intervening between the plates that so 
often masks the laminated structm’e of the body of the tooth in the region of the pri- 
mary plates when seen in transverse section ; and this is especially the case in the centre 
of the tooth, where these reticulations are most complicated, and where the attenuated 
angle of the plate in its most advanced state of development favours the obscm’ation of 
the layer-like appearance. 
At first this moss-like growth consists principally of narrow fibres, but soon the 
extremities spread out into broad fiattened expansions, club-shaped, fan-shaped, and of 
infinite diversity of form (Plate VI. fig. 7, k, I, n, p). It is these expansions at the ex- 
tremity of the reticulations, extending beyond the margin of the plates, that constitute 
the coarse tubular tissue at the sides of the keel near the body of the tooth, and at the 
enteric margin of the body itself. 
