OF THE NORFOLK " FOKEST-BJiD." 317 



and the entire tooth is flattened from side to side, particularly in^ 

 the region of the pulp-cavity, the flattening being natural and not 

 due to post-mortem compression. The greatest thickness of the- 

 tooth is at a little more than a third from the apex, and from this 

 region to the base it becomes rapidly compressed. The large size of 

 the pulp-cavity and tlie entire absence of globular osteodentine 

 show that the tooth was not fully developed. The proportion of 

 dentine to cement is shown in the section (PI. XI. fig. 2). At the 

 middle of the tooth, where the larger diameter is 2-35 in., the den- 

 tinal core has a diameter of 1-7 in., and the cement is about 0*3 in. 

 thick on each side. 



Measurements. 



inch. 



Greatest length of tooth 5-8 



Greater diameter, one third from base 2*5 



Lesser diameter, one third from base 1-6 



Greater diameter, one third from apex 2-2 



Lesser diameter, one third from apex 1'75 



Greater diameter at base 2-6 



Lesser diameter at base 1*3 



Pulp-cavity extends from base 2-65 



Thin sections examined with the microscope show the dentinal 

 tubules passing obliquely upwards from the axis with slight undu- 

 lations, and giving off at intervals fine anastomosing branches ; just 

 before reaching the cement layer the tubules become finer and ter- 

 minate in the granular layer, a more opaque band in which the 

 structure is obscure. Throughout the dentine, and especially to- 

 wards the peripheiy, the globular calcigerous cells are more distinct 

 than is usually \\iq case in mammalian teeth ; but this peculiarity 

 seems to be shared by other forms of marine mammals. 



The minute characters of the cementum are remarkable, and 

 although repeated, in a less marked degree, in some recent forms, 

 they do not seem to have been fully described. Polished sections of 

 the cement, even when examined with a pocket lens, show series of 

 laminae, and under the microscope thin sections show these to be 

 composed of alternating, more or less transparent bands. In a 

 transverse section, besides the lacunae, which are arranged on the 

 whole parallel to the lamina), there are numerous " Sharpey's fibres" 

 which cross the bands. For the most part, these fibres are far 

 larger than the tubules of the dtjntine, and much less regular in size 

 and direction ; passing from the inner side they undulate, crossing 

 and recrossing one another so as to produce a complex plaited 

 appearance. In a longitudinal section the undulations are seen to 

 be much more deeply curved (fig. 3, h to c). The fibres do not remain 

 of the same size throughout, but are enlarged at intervals, and be- 

 coming more opaque produce the appearance of spindle-shaped bodies. 

 When these bodies are examined with a power of 200 diameters or 

 more (fig. 4) they may be seen to contain one, two, or perhaps three 

 rows of indistinct spherules, which are probably calcigerous cells. The 



