^°r.' EVOLUTION OF SHARKS' TEETH. 673 



In short, compound cuspidate teeth in sharks are always uniserial, 

 and owe their origin to the constant and regular fusion of primitively 

 separate, adjacent, simple cusps. 



II. — The Fusion of Successional Teeth. 



In accounting for the complication of cuspidate teeth in mammals, 

 however. Prof. W. Kiikenthal informs the writer that he is inclined to 

 proceed beyond stating that such teeth result from the fusion of simple 

 adjacent cusps. He even suggests that a successional set of cuspidate 

 teeth may fuse completely with the set of fully-formed teeth already 

 in front — that in such primitive mammals asPolymastodon, for instance, 

 each tooth may be a compound of cusps derived from both the first and 

 second sets, which normally develop separately and one after the 

 other. 



It is of great interest to note that precisely the same phenomenon 

 can be observed in a certain specialised group of extinct sharks, 

 characteristic of the latter part of the Palaeozoic epoch. The family 

 of Cochliodonts, of which little is known except the dentition (1-6, 8), 

 is so named from the great shell-shaped plates into which its 

 principal grinding teeth have more or less completely coalesced ; and 

 all circumstances point to the conclusion that these simple plates are 

 the equivalent of several distinct rows of successional teeth in the 

 ordinary sharks with grinding teeth, as represented by the living 

 Cestyacion. 



The Port Jackson Shark, or Cestyacion, as is well known, exhibits 

 two principal series of grinding teeth on the middle of each half of 

 the jaw (Fig. i), in which there are five or six successional teeth 

 behind (or within) the foremost row. In Psephodiis (Fig. 2) the equiva- 

 lent of the hinder of these two series forms a great simple plate ; and 

 this is shown by abnormal specimens (Davis, No. i., pi. Iv., fig. 4) 

 to consist of three rows in succession. In Pletivoplax or Pleurodus 

 (Fig. 5), Deltodus (Fig. 6), and Cochliodus (Fig. 3), each of the two 

 series is fused into a continuous plate, which always distinctly 

 exhibits traces of its components in the first two genera. In Pacilodiis 

 (Fig. 7) and Deltoptychius (Fig. 4), not only do the two plates occur, 

 but they are once more fused together at their adjacent border into 

 one continuous piece. The coalescence of the teeth is so complete in 

 all cases, that their mode of growth is much changed ; the once- 

 separate outer tooth can no longer fall away from the mouth, as 

 growth of tooth-substance takes place within, and the external border 

 thus coils inwards like a scroll, as shown in Fig. 8. 



Briefly, then, compound crushing dental plates in sharks result 

 not only from the fusion of adjacent teeth, but also often from the 

 coalescence of several successional teeth. 



III. — The Multiplication of Similar Cusps. 

 Fusion of cusps and plates, however, will not explain all the 



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