OCR Text |
Show 1906.] TEETH OF CREODONTS. 49 it is difficult to see the outline of the enamel prisms, which, however, where traceable pursue a nearly straight course. When, however, w e come to the enamel of Thylacinus (text-fig. 13), we find that the tubes thin out and are lost before they reach the exterior of the enamel, even where this is thickest, though sections may be found in which they penetrate further Text-fig. 13. Thylacinus.-Longitudinal section of dentine and enamel. The tubes reach halfway through the enamel. Text-fig. 14. Basyurus.-Longitudinal section of dentine and enamel. than in that figured. And where there is a tolerably abundant passage of tubes, the so-called granular layer, which marks the exterior of Placental dentines {cf. text-fig. 15, p. 50), is absent or but little conspicuous. Outside the region of the tubes the enamel prisms are fairly distinct and are seen to be straight; the straight prisms m a y also be traced by careful illumination right in to the PROC. ZOOL. Soc-1906, YOL. I. No. IY. 4 |