THE ANATOMY OF PLBQKOTOMABTA BEYEIOHII. 251 



a decided reduction in the length of the hooked portion of 

 the tooth, the three cusps being now somewhat closely 

 crowded near the free end of the tooth. The tuft of bristles 

 now appears to be inserted lower down on a ridge placed 

 transversely to the axis of the tooth, and the bristles them- 

 selves, forming a compact brush, extend considerably beyond 

 the free end of the tooth (figs. 50 — 52). With the reduction 

 of the hooked portion of the tooth the two sets of bristles meet 

 behind, and now form a horseshoe-shaped brush embracing 

 the end of the tooth. Passing outward the teeth become 

 still more delicate, and the cusps smaller and smaller, until they 

 completely disappear. Traces of the cusp-bearing lobe are, 

 however, still distinctly visible on the 101st tooth, although 

 it is now only a narrow slightly notched process. The same 

 structure, but still smaller and devoid of notches, maybe seen 

 on the 102nd and 103rd teeth, and I think on all the 

 remaining teeth, in the form of a slight process on upper 

 border of the teeth. 



As the upper tooth-bearing lobe becomes reduced, the two 

 sets of bristles run together and form a single clump, and 

 gradually approach the free uppei- border of the tooth. This 

 latter condition is accelerated by the development on 101st — 

 104th of a lamina springing from the back of the tooth, and 

 foreshadowing the flabelliform tooth (fig. 53). 



The bristles remain well developed after the disappearance 

 of the cusps, and even the 103rd tooth possesses a good brush. 

 The 104th, however, shows a marked reduction in its bristles, 

 and this is the last of the brush teeth, since the 105th tooth 

 is entirely devoid of these structures. In other respects the 

 difference between these two teeth is slight, their general 

 form being very similar. 



The Flabelliform Teeth. — There are seven of these 

 teeth (the 105th to the 111th inclusive), which have the form 

 of delicate narrow lamellfe, arranged like the rays of a fan ; 

 they all bear a slight notch at their free end, corresponding 

 to the point of attachment of the bristles in the brush teeth, 

 and possibly representing the tooth-bearing lobe. 



