TOOTH-DEVELOPMENT IN ORNITHORHYNCHUS. 163 



as they go, the facts rather point in the direction of serial 



continuity between the teeth of the molar series and their 



putative serial companions in front, meagrely represented in 



" V — w " 

 our older specimen " Beta by the teeth rr-r\ r,- 



^ -^ " (v) — w" 



Finally, in several of the figures illustrative of the condition 

 in the older specimen (figs. S, 9, 10) it will be observed that 

 a tolerably substantial ''residual dental lamina" remains pre- 

 served along the lingual side of the large molar enamel-oi-gans. 

 We have not so far had an opportunity of observing the sub- 

 sequent fate of this " residual lamina/' but there is no reason 

 for believing that it differs in any way as regards its destiny 

 from the residual lamina so often found in a similar situation 

 beside the developing molars of other mammals. Its presence 

 here, therefore, need not be regarded as expressive of any 

 special significance beyond that of representing the formal 

 potentiality of "post-permanent" molar successors, the pos- 

 sibility of the occurrence of which cannot be excluded from 

 the ordinary mammalian scheme of tooth-genesis. 



List or References. 



1. PouLTON, E. B.— " True Teeth in the joung Ornithorhynchus para- 



doxus," *Proc. Roy. Soc.,' vol. xliii, 1888, pp. 353-356. 



2. PouLTON, E. B.— "The True Teeth and the Horny Plates of Orni- 



thorhynchus," ' Quart. Journ. Micr. Sci.,' N.S., vol. xxix, 1889, 

 pp. 9-48. 



3. PouLTON, E. B.— "The Structure of the Bill and Hairs of Orni- 



thorhynchus paradoxus: with a Discussion of the Homoloo-ies 

 and Origin of Mammalian Hair," 'Quart. Journ. Micr. Sci.,' N.S., 

 vol. xxxvi, 1894, pi. xvtf. 



4. Thomas, 0.— "On the Dentition of Ornithorhynchus," 'Proc. Roy. Soc.,' 



vol. xlvi, 1890. 



5. Stewart, C— " On a Specimen of the True Teeth of Ornithorhynchus," 



'Quart. Journ. Micr. Sci.,' N.S., vol. xxxiii, 1892, pp. 229-231, 

 pi. viii. 



6. Mauett Tims, H. W.—" Tooth-Genesis in the Caviids," 'Journ. Linn. 



Soc. Zool.,' vol. xxviii, 1901, pp. 261-286. 



7. Marett Tims, H. W.— "The Evolution of the Teeth in the Mammalia," 



'Journ. Anat. and Phys.,' vol. xxxvii, 1903, pp. 131-149. 



