152 J. T. WILSON AND J. P. HILL. 



a photomicrograph of one of the epithelial strands, in which 

 its real nature as a prolongation of the epithelium of the tip 

 of the cusp is manifest. High-power examination lias also 

 shown us, in the case of a few sections through such a strand, 

 that the inner enamel-epithelium is actually prolonged in the 

 core of the cylinder in the form of two closely-applied layers 

 of cells. Between these layers aline, indicative of a potential 

 lumen, is just recognisable, and we regard the cylinder as an 

 abortive prolongation of the apical part of the pulp-cavity, 

 within which, pulp-cells have failed to penetrate, or have wholly 

 disappeared, so that neither dentine nor odontoblasts have 

 been differentiated within it. The epithelial cylinders thus 

 represent, in our view, portions of the cusps which have 

 undergone ontogenetic reduction. 



(2) Position and arrangement of nodules with 

 reference to cusps. — Poulton states that there was 

 always a nodule over the apex of each of the chief cusps of 

 the two large teeth, while they were never found elsewhere. 

 We likewise find a nodule in relation wnth each of the two 

 chief cusps of these teeth — i. e., over the series of internal 

 cusps in the upper, and of the external cusps in the lower jaw 

 (text-fig. 2). 



The photomicrographs herewith reproduced in figs. 7, 8, 

 and 10 illustrate the topographical relationship of several of 

 the chief cusps to their corresponding nodules as seen in the 

 section-series from one side of our specimen ''Beta." In 

 figs. 8 and 10 the nodules appear to lie inside the enamel- 

 organ, whilst in fig. 7, as already stated above, it only 

 partially indents the superficial aspect of the enamel- organ 

 {of. also fig. 6 from opposite side). 



It will further appear from an examination of text-fig. 2, 

 which also illustrates the condition met with on the same side 

 of the head, that in the case of the anterior chief cusp of the 

 enamel-organ " y," there are two nodular structures present 

 in the vicinity of the cusp. The more anterior of these is 

 seen in the photomicrograph in fig. 8. It pi*esents the typical 

 characters of the nodules as described by Poulton and as we 



