164 J. T. WILSON AND J. P. HILL. 



8. Wilson, J. T. and Hill, J. P. — " Observations upon the Development 



and Succession of the Teeth in Peranieles, etc.," ' Quart. Journ. Mier. 

 Sci.,' N.S., vol. xxxix, 1896. 



9. Wilson, J. T. — " Description, with figures, of a young specimen of 



Ornithorhynchus anatinus from the Collection of the Australian 

 Museum, Sydney," ' Proc Linn. Soc, N.S.W.,' scr. 2, vol. ix, p. GS2. 



EXPLANATION OF PLATES 10-12, 



Illustrating Professors Wilson and Hill's paper, " Observa- 

 tions on Tootli-Developmeut in Ornitliorliynclius.^' 



All the figures are from photomicrographs of transverse sections. The 

 symbol tIt indicates the labial side. 



Fig. 1. — Delta. Section of the enamel organ and dermal papilla of tooth 

 " X " of the upper jaw, right side, with nodule rudiment on the labial side of 

 neck of enamel organ, x 78. 



Fig. 2. — Delta. Tooth " x " of left side, upper jaw, also with a labially- 

 situated nodule rudiment. In the lower jaw tlie dental lamina is swollen club- 

 like to form the rudiment o f the lower tooth "y." x 78. 



Pig. 3. — Delta. Tooth "w" of lower jaw, right side. Note the narrow 

 pointed character of the dermal papilla as seen in cross-section and compare 

 with that of " x" as seen in figs. 1 and 2. On the lingual side the residual 

 dental lamina is commencing to free itself from the enamel organ. X 78. 



Fig. 4. — Delta. Section through the dental lamina of the upper jaw, left 

 side, about mid-way between its anterior extremity and the enamel organ " w," 

 showing the rudimentary toothlet "dv " indenting the oral epithelium to the 

 labial side of the attachment of the lamina. X 175. 



Fig. 5. — Delta. Corresponding section through the vestigial toothlet " dv " 

 on the right side of the upper jaw. x 175. 



Fig. 6. — Beta. Section through the upper tooth " x " passing through its 

 main antero-internal cusp and the nodule " dx^ " related thereto. X 78. 



Fig. 7. — Beta. Corresponding section showing the tooth "x" and the 

 nodule " dx* " of the opposite side of the head. X 78. 



Fig. 8. — Beta. Section through the antero-internal cusp, upper tooth- 

 "y," and its related nodule "dy'." x 78. 



Fig. 9. — Beta. Section shortly behind the preceding, showing the tootii- 

 lef'dy'" underlying the posterior region of the antero-internal cusp of upper 

 " y,"and well to the labial side of the residual dental lamina. The toothlet lies 

 directly behind and in series with the nodule "dy'" (cf. text-figure 2). x 78. 



