REPORT ON THE CETACEA. 19 



a sinuous slightly raised line, half an inch from the apex, seemed to mark off the crown 

 from the fang, and probably indicated where the gum had embraced the tooth. The 

 outer surface of the tooth was smooth, except near the base, which was marked longi- 

 tudinally with shallow furrows ; it was of a dull whitish-yellow colour. 



About the middle of the elongated base was a narrow chink, which had evidently at 

 one time extended along its whole length, but in course of time had become almost 

 entirely occluded. When a vertical section was made through the middle of the tooth, 

 this chink was seen to communicate with a pulp-cavity, which extended to 4-lOths of an 

 inch from the apex of the tooth. Near the base of the tooth the cavity was so contracted 

 that the opposite walls were almost in contact, but in the middle of the tooth it dilated 

 into a well-marked cavity (PI. III. fig. 20). 



A thin vertical section was then cut out of the tooth from base to apex, and prepared 

 for microscopic examination. Under a low power the tooth was seen to consist in its 

 greater part of dentine, which formed the wall of the whole of the pulp-cavity, except at 

 the basal end of its contracted portion. In the upper third of the tooth the dentine tubes 

 radiated in a very regular manner from the pulp-cavity outwards, but in the lower two- 

 thirds, they were broken up into clusters and tufts, and sometimes irregularly scattered 

 throughout the dentine matrix. The surface of the section through the dentine was 

 marked by contour lines parallel to its surface, which expressed the primary curvatures 

 of the dentine tubes (fig. 20). But, in addition, a line of interglobular spaces lay in the 

 substance of the dentine, parallel to these contour lines, and about midway between 

 the most external of them and the exterior of the dentine. This line of interglobular 

 spaces did not pass in one direction much beyond the apex of the pulp-cavity, but in the 

 other it extended some distance into the fang. 



The dentine in the crown was invested by a thin layer of substance, which had the 

 position and relations of a layer of enamel to the dentine. It extended as far down the 

 tooth as opposite the apex of the pulp-cavity, where it was overlapped by the cement, but 

 at the very tip of the tooth it was absent, having apparently been worn off. The charac- 

 teristic enamel structure was not so definite in it as in the corresponding layer on the 

 crown of the young tooth of Mesoplodon layardi, but in thin sections it was seen to be 

 traversed by fine lines extending perpendicularly to the surface of the tooth, which 

 obviously indicated the direction of the rods of enamel. But the exterior of the crown 

 did not have the brilliant white appearance so characteristic generally of the enamel. 



The free surface of the fang was invested by a thin but definite layer of cement. 

 Where the dentine was covered by the cement, a change in the structure of the dentine 

 occurred. Vascular canals were seen to lie in it perpendicular to the free surface of the 

 tooth, and forming loop-like curves immediately subjacent to the cement. This portion 

 of the dentine was, therefore, a vaso-dentine. As the cement and vaso-dentine were 

 traced lower down in the fang, other modifications became aj^parent. The vaso-dentine 



