GEOWTH OF THE TOOTH OF ECHINUS. 
407 
PLATE VIII. exhibits the ultimate histology of Echinus-tooth. 
Fig. I. Portions of four plates studded with “soldering particles;” also reticulations of 
the flabelliform processes protruding between the plates, and likewise dotted 
with soldering particles. Magnified 200 diameters. 
Fig. 2. A few soldering particles adherent to a portion of plate. Magnified 200 dia- 
meters. 
Fig. 3. Soldering particles adherent to a portion of plate, but extending beyond its 
limits, and retained in relative position by their connecting film. Magnified 
200 diameters. 
Fig. 4. A sheet of soldering particles, entirely detached from then* plate, but held 
together by their connecting film. Magnified 200 diameters. 
Fig. 5. Other soldering particles : one seen edgewise ; J, three connected particles — 
one produced by fusion of three smaller ones ; c, large particle, the result of 
the fusion of four ; f?, small particles with very distinct single centres ; large, 
nearly homogeneous particles. Magnified 200 diameters. 
Fig. 6. Enamel on the extreme dorsal surface, as seen in transverse section, exhibiting 
its tubes, some containing bubbles of air. Magnified 400 diameters. 
Fig. 7. Vertical section of keel, showing the general linear arrangement of the lacunae, 
coincident with the oblique direction of the fibres of the keel. Magnified 
100 diameters. 
Fig. 8. Vertical section of keel, where just sufficiently condensed to admit of being 
groimd into section, displaying the same general linear arrangement of lacunae 
as in fig. 7, — the canaliculi mainly passing at right angles to that serial 
ari’angement. At a the soldering particles are indicated, with the interstitial 
lacunae ; at h the ends of the fibres in their extreme vertical breadth and with 
their ciystalline fracture. Magnified 400 diameters. 
The whole of the figures in these Plates have been taken from specimens prepared by 
the author ; and ufith one exception (fig. 8, Plate VIII.) they have been drawn, to 
ensure exactness, with the assistance of the camera lucida. 
