TEETH OF PHYSETBR 29 



the interdental regions by its less swollen character. Pre- 

 cisely the same is to be seen in the lower jaw, where (Text-figs. 

 2-4) the dental rudiments are definite outgrowths of the dental 

 lamina, which is, however,, a continuous outgrowth, being 

 merely thinner in the interdental intervals (Text-figs. 2, 3). 

 This will, I think, be made plain by an inspection of the figures 

 referred to. There is to be seen, as I interpret the facts ascer- 

 tained and figured by Messrs. Wilson and Hill (8, p. 141, 

 Text-fig. 1), a perhaps comparable state of affairs in the 

 developing teeth of Ornithorhynchus. 



In the younger of two foetuses examined by those two 

 anatomists the entire dental lamina of both upper and lower 

 jaw (of one side) is figured and described. From those figures 

 and the descriptions it is to be inferred that the enamel organs 

 of two teeth are differentiated in the substance of the lamina 

 of each half-jaw as a thickening of it, and not as an outgrowth 

 therefrom — the connecting part of the dental lamina remaining 

 unaltered between those rudiments. This is, as I think, to be 

 compared exactly with any two succeeding teeth of the upper 

 jaw of Physeter, where the tooth thickening is merely the 

 lamina itself, and the unaltered lamina remains in the same way 

 between successive tooth germs. This — as it will not be for- 

 gotten — is different from the lower jaw teeth of Physeter; 

 for these are outgrowths of the dental lamina in the form of 

 a continued laminal outgrowth thickened at intervals to form 

 the actual teeth rudiments, which remain connected by the 

 unaltered laminal outgrowth. 



The fusion between successive teeth in this the youngest 

 stage of Physeter as yet known may have some bearing upon 

 the theory of tooth origin, i. e. as to whether separate teeth, 

 like those of Physeter, are primitive, or bIiow signs of the 

 breaking up of a complex tooth series. Are the unions between 

 the individual teeth a promise of a later concrescence, or the 

 remains of a separation of the cusps of a complex tooth '? I have 

 not, however, been able to ascertain any further facts which 

 bear upon this most interesting topic. I can, for example, 

 see no gaps which might mark the boundaries of pre-existent 



