406 
ME. S. J. A. SALTEE ON THE STEHCTUEE AND 
Fig. 7. These details represent the elementary fibres of the keel, and the flabelliform 
processes, whose dilated ends form the skirtings of the tooth ; from a to li are 
shown difierent degrees of development of the keel-rods, or fibres: on the 
surface of li the soldering particles are seen in their earliest condition: 
i represents the broad fiattened end of a large fibre ; k, I, different forms of 
keel-fibres and flabelliform processes ; wz, keel-fibres detached from their plate, 
but showing mutual linear adhesion at their proximal extremities ; n, flabel- 
liform processes exhibiting the same; o, keel-fibres still attached to plate; 
]), flabelliform processes displaying the same condition. These details are 
variously magnified 150 and 75 diameters, as indicated in the Plate. 
PLATE VII. 
Fig. 1. Plates viewed on the external or dorsal surface. Magnified 100 diameters. The 
part of the tooth here represented corresponds with the lower portion of fig. 6, 
Plate VI., but is from a much larger tooth. 
Fig. 2. Plates of a large tooth, seen on the enteric or ventral surface, and showing the 
progressive growth of the “ secondary plates.” The plates are artificially 
arranged at intervals oi Jives, so as to exhibit more conspicuously the change 
that occurs. The figure indicates the progressive development in seventy 
plates. Magnified 100 diameters. 
Fig. 3. Ventral or enteric surface of Echinus-tooth in an advanced state of development. 
The secondary plates are recognized on either side by their mamilliform pro- 
cesses : the central straight and lateral converging acicular fibres of the keel 
are also displayed. The lines of intersection of the inferior edges of the pri- 
mary plates at their dorsal aspect are seen looming indistinctly out of focus. 
Magnified 50 diameters. 
Fig. 4. Primary plates viewed on dorsal surface in an advanced state of development, 
displaying their modified form, the incremental lines at their outer angles, and 
the commencement of the enamel rods on their lower margins, uncovered 
by the imbrication of the contiguous plates. Magnified 100 diameters. 
Fig. 5. Transverse section of keel, showing its shell-like appearance when thus displayed. 
Magnified 200 diameters, a exhibits the different forms of cut surface as the 
vertical depth of the fibres increases, while the lateral breadth remains the 
same ; h, the same, where a fracture has isolated the ends of several fibres. 
