GEOWTH OF THE TOOTH OF ECHINUS. 
399 
The secondaiy plates are developed from the outer margin of each series of primary 
plates on either side, and in their growth project towards the enteric region and towards 
the mesial line, approaching their fellows of the opposite side. When seen in situ and 
in their complete development, the mamilliform processes present a beautiful even series, 
between which the rods of the keel pass inwards. This is displayed in Plate VII. fig. 3. 
Woodcut II.* 
The angle at which the secondary bends away from the primary plate can be best 
estimated by contemplating a section through their centre — the average presented in a 
transverse section of a mature tooth (Plate VI. fig. 4), and which is diagrammatically 
represented in Woodcut I. a, 5, and Woodcut II. The secondary plates are adherent to 
the outer edge of the primary, but they easily break off from them, coming away entire 
— the fracture being sharp and complete along the line of attachment. It is very 
common, in breaking up a tooth wdth needle points under the microscope, to see the 
secondary plates floating about free, but themselves unbroken : they may always be recog- 
nized by their characteristic mamilliform processes. The secondary plates are of the 
same nature as the primary — thin sheets of carbonate of lime, and they break up witli 
exactly the same crystalline fracture f. 
Keel Bods or Fibres . — These elements of the tooth constitute a very large proportion 
of its bulk — larger than any other singly. They consist of certain fibres which origi- 
nate from the superior angle of the primary plates, attached to their extreme edge, and 
projecting inwards loose and free towards the enteric region. The source of these 
fibres, commencing in the centre where the two sets of plates interlock, gradually 
extends laterally, — the central fibres being the oldest and longest, those proceeding out- 
wards in their attachment, shorter and shorter. The central fibres pass straight dowii- 
* Diagram showing the relation of a single Primary and Secondary Plate, a, Primary Plate with 
Secondary Plate folded on it ; the line across them presenting the section figured at h. The latter has 
been turned round so as to coincide with the sectional lines a and J in W oodcut I. 
t It is remarkable that neither Valentin nor AVilliamson appears to have been aware of the existence 
of these secondary plates, though they are sufficiently conspicuous, aud constitute very important items in 
building up the fabric of the tooth. 
