142 J. T. WILSON AND J. P. HILL. 



emancipation from them of a " residual dental lamina '^ or 

 " Ersatzleiste " (PI. 10, figs. 1, 2, 3). 



The two enamel-organs in each jaw occupy very unequal 

 lengths of the dental lamina. As shown in text-fig, 1, the 

 anterior of the two enamel-organ (" w ") is considerably less 

 elongated antero-posteriorly than the one behind it ('^x"). 

 There is also a slight difference in height in favour of the 

 posterior enamel-organ, but there is very little difference 

 between their dimensions measured in the coronal plane — i.e. 

 transversely to the length of the dental lamina. The form of 

 the anterior enamel-organ further shows a difference from 

 that of the posterior one. Its papilla is relatively more elon- 

 gated and is more jDointed than that of the posterior enamel- 

 organ. This characteristic of the anterior tooth-germ is 

 illustrated (for the lower jaw) in fig. 3. In this figure it 

 will be seen that the '' residual dental lamina" (''Ei'satz- 

 leiste ") is just beginning to emancipate itself, by further 

 growth, from the medial aspect of the enamel-organ. In 

 front of the anterior enamel-organ, in each jaw, the dental 

 lamina is prolonged for a considerable distance as a narrow 

 ridge of no great vei-tical height (text-fig. 1). In one place 

 it exhibits a small localised thickening {" dv ") lying about 

 midway between the anterior enamel-organ and the plane 

 of the anterior disappearance of the much reduced lamina. 

 Apart from this slight bulbous thickening, the cross-section 

 of this anterior segment of the dental lamina exhibits a 

 slightly flask-shaped, though attenuated form, right up to 

 its point of disappearance anteriorly. It therefore here 

 lacks the broad and shallow character which one is accus- 

 tomed to associate with the true and original anterior, or 

 rostral, extremity of the dental lamina. Its actual form here 

 is suggestive rather of the suppression of a formerly more 

 extensive anterior segment of the lamina. Nor is this merely 

 a surmise. For, in the course of this attenuated anterior 

 segment of the lamina, lying in front of the more anterior of 

 the two evident enamel-organs, we find a calcified vestigial 

 tootlilet. This is present in both jaws in our series of thin 



