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



are about seven teeth with single cusps which correspond to the outer 

 cusps of the posterior part of the series. In this genus there is a longi- 

 tudinal channel between the cusps of the molar teeth. Seen from the 

 palatal aspect the crown of a tooth is sub-quadrate, and the external 

 cusp is the more elevated, so that the tooth has an aspect which is 

 insectivorous rather than edentate. Both cusps are compressed so as 

 to form sharp longitudinal cutting edges. At their bases they are 

 connected on both the anterior and posterior borders of the tooth by 

 low transverse concave ridges. In my specimen these transverse 

 ridges are sufficiently marked in the skull ; but are stronger in the 

 lower jaw, where their surfaces are not quite smooth. If the anterior 

 and posterior ridges were stronger, the crown of this tooth in quadrate 

 form, external and internal cusps and elevated border, would be suffi- 

 ciently similar to the tooth of Omithorhynchus to give some ground 

 for regarding that tooth as reptilian in plan. And it has already been 

 seen that in degeneration of the fang, which induced Sir R. Owen to 

 compare the teeth to those of the reptile genus Placodus, and in 

 implantation in a groove in the jaws there is no departure from 

 reptilian types. If the tooth of the Omithorhynchus as a whole 

 cannot be exactly paralleled in any other animal, it is at least evident 

 that the teeth are as reptilian as the skeleton ; and if the corre- 

 spondence is not closer, the reason may be found in the degeneration 

 which has replaced the enamel of the tooth with horny matter. 



Modern lizards are not a group of animals in which theoretical 

 considerations would suggest a search for mammalian characters in 

 the teeth, but they happen to be the only group of Reptilia which is 

 at all well known in which the teeth show a diversity which is in any 

 degree comparable with the diversity of mammalian teeth. Whether 

 those characters have been inherited from remote ancestry, or spon- 

 taneously developed in their possessors under varying conditions of 

 existence, as seems probable, is a matter of small moment, for in 

 either case they throw illustrative light on the classificational value of 

 teeth of mammalia. If the different forms of cusp development found 

 in lizards could be combined, teeth would result with crowns like the 

 cuspidate crowns of many mammals. Thus, in Cnemidophorus there 

 are two cusps arranged longitudinally ; in Ameiva the tooth may 

 have one large cusp with a small cusp by its side, or in the fore-part 

 of the jaw there may be a small cusp on each side. If this kind of 

 serration were combined with the transversely bicuspid teeth of Teius 

 or of Empedias crowns would result which would have mammalian 

 patterns. In Amblyrhynchus cristatus the external part of the crown 

 is deeply cleft so as to be divided much as in some seals into a median 

 denticle, flanked by a lateral denticle on each side; but on the internal 

 side the base of the crown thickens, forming a large flattened oblique 

 area, which is evidently an undivided internal cusp, like the internal 



