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



but this condition of teeth varies in every order with the habitual 

 food. 



Finally, the succession of the teeth has been regarded as a mam- 

 malian class character. It is exceptional, and an individual peculi- 

 arity, for more than two sets of teeth to be cut in a mammal, though 

 evidence has been brought forward that this reptilian condition is 

 occasionally present in man. But even in those mammals which cut 

 a second set of teeth there are commonly some molars which have 

 ho predecessors, and are a single series throughout life. So far as is 

 known, most Edentata and Cetacea have but one set of teeth, which is 

 never renewed; and according to Professor Flower, Tatusia is the 

 only Edentate in which successional teeth are known to be developed. 

 I have seen no evidence of a successional tooth in any Dicynodont 

 reptile. Sir R. Owen has found no evidence that the Theriodontia 

 possessed " a milk series of teeth."* When a successional tooth is 

 present in mammals it usually originates below the tooth in wear, or 

 behind it as in the elephant. This condition is seen in some reptiles 

 as iu the Ornithischia. But the typical condition of reptilian succes- 

 sion is for the germ of the new tooth to be on the inner side of the tooth 

 in wear. This is the condition in Ichthyosaurs and most of the extinct 

 Reptilia, and is often though not invariably seen in Crocodiles. It is, 

 therefore, interesting that Mr. Poulton describes the new-found teeth 

 in Ornithorliynchus as possibly on the inner side of the so-called 

 horny plates, though in the lower jaw they are certainly below those 

 plates. Hence, if those germs are successional teeth their relative 

 position would not be inconsistent with reptilian or mammalian 



type. 



From this discussion I conclude that in all morphological relations 

 the teeth of mammals may be so simplified as to approach closely to 

 conditions which would be regarded as typically reptilian. 



I have next to show that the prevalent conception of the reptilian 

 type of tooth is equally indefinite. The differentiation is less striking 

 than among mammals, but in almost all morphological characters 

 reptiles suggestively approach mammals, though these characters 

 seem to me most remarkable in the grouping of the teeth into ana- 

 logues of molars, premolars, canines and incisors, and in the 

 characters of the crown in molar and other teeth. It is rather among 

 the oldest extinct Reptilia that we should expect to find the nearest 

 approach to mammalian dentition, and so it is ; but evidences of a 

 similar differentiation may be detected among Crocodiles and Lizards. 



The form of the crown varies very little from front to back among 

 Crocodiles, though some teeth are relatively large, and the smaller 

 posterior teeth are a little compressed transversely ; but when the 

 teeth are drawn from the jaw the alveoli show modifications which 

 * ' Geol. Soc. Quart. Journ.,' vol. 37, p. 261. 



