REPOET ON THE CETACEA. 45 



Fig. 16. Vertical transverse section through the same tooth to show the pulp cavity. 

 Natural size. 



17. Vertical transverse section through the upper part of the shaft of the tooth 



of the adult Mesoplodon layardi. The right shaded portion is the outer 

 surface of the tooth, from the upper end of which the denticle projects. 

 Natural size. 



18. A magnified vertical transverse section through the tooth of the young 



Mesoplodon layardi. P, pulp cavity; e, enamel ; d, dentine ; c, cement ; 

 V, modified vaso-dentine on exterior of dentine ; v', modified vaso-dentine 

 lining the pulp cavity ; i, the interglobular spaces. Page 11. 



19. A magnified vertical transverse section through the summit of the shaft of 



the tooth of the adult Mesoplodon layardi. a, Cement ; h, the subjacent 

 modified vaso-dentine ; c, a layer of more opaque modified vaso-dentine ; 

 d, dentine ; e, modified vaso-dentine of centre of shaft. The peripheral 

 canal on the surface of this layer between it and the dentine is shown. 

 As this drawing was not made until some months after the description 

 was written, during which time the balsam used in mounting the section 

 had contributed to make it more translucent, Mr Young was enabled to 

 recognise and figure a faint lamellation about the canals of the central 

 layers, as well as to see the passage of an occasional canal from the 

 layer h into the dentine, through which they probably extended obliquely 

 into the layer e. Where h and e blended .with each other, the passage 

 of canals directly from one to the other was seen. Page 14. 



20. Vertical transverse section through the tooth of Mesoplodon soiverhyi. 



Natural size. 



21. A magnified vertical transverse section through the upper half of the same 



tooth, e. Layer of ill-defined enamel ; d, dentine ; c, cement ; v, vaso- 

 dentine ; mv, modified vaso-dentine. P, pulp-cavity. Page 19. 



22. A similar magnified section through the lower half of the same tooth. P, 



pulp-cavity ; d, dentine ; mv, modified vaso-dentine ; c, cement. Page 20. 



(ZOOL. CHALL. EXP. — PART IV. —1880.) D 7 



