TOOTH OF ECHINUS. 



501 



out. The complete Tooth has somewhat the form of that of the 

 front tooth of a Rodent ; save that its concave side is strengthened 

 by a projecting 'keel,' so that a transverse section of the tooth 

 presents the form of a J_. This keel is composed of cylindrical 

 rods of Carbonate of 'Lime having club-shaped extremities lying 

 obliquely to the axis of the Tooth (Fig. 291, a, d) ; these rods do 



Fig. 291. 



Structure of the Tooth of Echinus: — a, vertical section, show- 

 ing the form of the apex of the Tooth as produced by wear, and 

 retained by the relative hardness of its elementary parts ; a, the 

 clear condensed axis ; b, the body formed of plates ; c, the so- 

 called enamel ; d, the keel : — b, commencing growth of the Tooth, 

 as seen at its base, showing its two systems of plates ; the dark 

 appearance in the central portion of the upper part is produced 

 by the incipient reticulations ,of the flabeLliform processes : — 

 c, transverse section of the Tooth, showing at a the ridge of the 

 keel, at b its lateral portion, resembling the shell in texture ; at 

 c, c, the enamel. 



not adhere very firmly together, so that it is difficult to keep them 

 in their places in making sections of the part. The convex surface 

 of the Tooth (c, c, c) is covered with a firmer layer, which has re- 

 ceived the name of 'Enamel ;' this is composed of shorter rods, 

 also obliquely arranged, but having a much more intimate mutual 

 adhesion than we find among the rods of the keel. The principal 

 part of the substance of the Tooth (a, b) is made-up of what may 



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