HORNY TEETH 367 



are in both Petromyzon and Bdellostoma horny structures 

 derived from the epithelium, and have no likeness or analogy 

 to calcified teeth. 



It has been considered that the teeth of the Cyclostomes 

 are vestiges of the calcified teeth of former types, but it is 

 now more generally held that they represent a stage in tooth 

 evolution. 



As Dr. Bridge says (3), ' The structure and development 

 of the teeth in the Cyclostomes lend no support to the view 

 that these teeth are degenerate calcified structures. With 

 greater probability they represent a stage in the evolution 

 of teeth and dermal spines, which has been succeeded by 

 a later stage in which calcification superseded cornification 

 as a method of hardening.' 



There is, in the first place, a downgrowth of the epithelium 

 forming a kind of tooth follicle, and beneath this a small 

 mesodermic papilla. The first indication of a forming tooth 

 occurs above this papilla, and the epidermal cells become 

 flattened. The tooth penetrates the superimposed cells and 

 appears on the surface, the indication of another tooth 

 being already present in a cornified layer of cells beneath it, 

 as shown in the figure taken from Warren's paper (fig. 243). 

 He draws attention to the close resemblance which this struc- 

 ture, with the mesodermal papilla below, bears to a develop- 

 ing hair. The figure shows that the tooth arises within the 

 corium, and is not formed from cells derived from the 

 mesoblastic papilla. 



The Teeth of Ornithorhynchus. Horny structures which 

 serve the functional purposes of teeth are seen in the Ornitho- 

 rhynchus ; but they are more correctly described as horny 

 plates, for, as will be seen, these are the plates in which the 

 true teeth of the animal are embedded, and are not developed, 

 as in the Cyclostomata, as independent dental structures. 

 For a long time these horny plates were considered to 

 represent the only functional teeth of Ornithorhynchus, until 

 in 1888 Professor Poulton(5) discovered true calcified teeth in 

 an embryo. From an examination of the available material 

 he concluded that the true teeth were beneath the horny 

 plates, but Oldfield Thomas in 1889 (7), from an examination 

 of other specimens, found that the calcified teeth were above 



