132 Prof. H. G. Seeley. On the Nature and Limits [Apr. 26, 



remainder of the tooth. I have had an opportunity, by the kindness of 

 Dr. Grarson, of examining the microscopic sections of these teeth pre- 

 pared by the late Professor Quekett, and preserved in the Museum of 

 the Royal College of Surgeons, and they confirm my previous impres- 

 sion that the central portion of the tooth is bony (at least in some 

 specimens), and in microscopic structure it shows large haversian 

 canals surrounded by spaces and canaliculi. I therefore regard these 

 teeth of Ornithorhynchus as true teeth. But they seem to me to be 

 teeth in course of degeneration, antl in process of losing their cal- 

 careous matter. They have already lost their root or roots, and haVvS- 

 partially lost their individuality. The long anterior dental ridges 

 appear to have carried this change one step further and have become 

 dental layers formed of vertical parallel plates of horn in which there 

 is no division into separate teeth, which are not imbedded in the jaw, 

 but are a horny superficial substance. It is not without interest to 

 remark that some other animals which have lost their teeth, like birds, 

 and presumably Chelonians, which use the jaws for biting, also have 

 them sheathed in horn ; for the condition in Ornithorhynchus suggests 

 that the horny substance may represent the lost substance of 

 teeth. 



Thirdly, mammalian teeth are commonly distinguishable into dif- 

 ferent kinds, which when fully developed vary in the forms of their 

 crowns, and are thus recognised as incisors, canines, premolars, and 

 molars. This differentiation is almost entirely absent from the denti- 

 tion of Cetacea and Edentata ; and it is well known that in different 

 orders, canine teeth, or incisor teeth, or both, may be absent. These 

 conditions can be frequently correlated with food. But just as the 

 grouping of the teeth in mammals may approach in simplicity the 

 condition in reptiles, so the teeth of some reptiles in different parts 

 of the jaws may parallel the divisions found in the jaws of mammals 

 which show considerable differentiation. 



The fourth mammalian character is the cuspidate condition of the 

 crown of the tooth. This results from a folding of the substance out 

 of which the tooth is formed, and among the molar teeth of many 

 mammals shows a specialisation which is unparalleled among reptiles. 

 But on the other hand the complexity of some hinder-molars becomes 

 simplified in the premolar region, and among Edentates and Cetaceans 

 the tooth crowns are simpler than among some reptiles. In several 

 orders of mammals it is obvious that the direction in which the folds 

 of tooth substance are disposed is at right angles to the direction of 

 movement of the lower jaw ; and therefore it may be a fair inference 

 that the transverse widening of molar teeth, no less than their diverse 

 cuspidate character, is to be attributed to the increased work which 

 food has given them to do in the molar region ; and that development 

 or suppression of a cusp in allied genera of mammals depends upon this 



