228 ' PKOCEEDINGS OF THE GEOLOGICAL SOCIETY. [Feb. 8, 



The tusks of Proboscideans are cylindrical, uncompressed, with- 

 out furrows, and generally much curved. Those of the Sirenia are 

 a great deal smaller, proportionately to the body, than these pro- 

 bably were ; they are also unfurrowed, nearly cylindrical, and almost 

 straight ; whilst the canines and incisors of the Hippopotami have 

 an entirely different form. 



Structure of the Tusks. — Probably one of the most satisfactory 

 methods of demonstrating the arrangement of the mineral matter 

 in teeth is by means of fossil specimens, since the complete destruc- 

 tion of their organic constituents, and the infiltration and absorp- 

 tion of various chemical substances, in most cases disintegrates 

 them, and developes or renders apparent a structure which would 

 otherwise escape observation. Thus the fossil tusks of the Mam- 

 moth easily split up into a series of superimposed hollow cylin- 

 ders ; the Cetacean teeth from the Red Crag often, owing to the 

 varying absorption of iron in the different tissues of the tooth, may 

 be broken into long laminse, such that the " crusta petrosa" or 

 " cement" is removed, and a very remarkable gyrate and striated 

 arrangement of the subjacent dentine is exposed. So with the fossil 

 tusks under description ; the exterior is, when properly preserved, 

 smooth, but marked by a number of tine longitudinal cracks, which 

 are most apparent in the small or young specimens. The lamina of 

 the tooth in which these cracks exist very frequently splits off, and 

 the subjacent surface is thereby exposed. A microscopical examina- 

 tion of this external lamina shows it to be the " cement " investing 

 the whole tusk, no enamel occurring in these teeth. When much 

 of the cement layer has been removed, as very frequently happens, 

 the soUd matter of the "dentine" which underlies it is seen to 

 have a very curious arrangement. Its surface, which becomes very 

 glossy and bright in these Crag fossils, is sculptured by a series of 

 small longitudinal grooves or furrows, producing a fluted ornamenta- 

 tion, whilst minute ridges or striations cross these at right angles, 

 the two series of markings together giving an appearance very simi- 

 lar to that observable on the surface of the canine of the Hippopo- 

 tamus (PI. X. fig. 1). 



This structure in the fossils under notice has led to their being 

 regarded as belonging to a form of Hippopotamus ; but inasmuch as 

 in that animal the grooves and striations are entirely superficial, 

 whilst in the Crag tusks, as also in Cetacean teeth of the same de- 

 posit, these markings are only displayed by the removal of the ex- 

 ternal lamina of cement, no weight can be attached to an assimilation 

 of the two forms of tusk based upon these grounds. 



The " dentine" of the fossil tusks has a tendency to split up into 

 long concentrically annular laminae, displaying, when removed by 

 the action of the sea or elsewise, a similar series of longitudinal 

 groovings ; so that this arrangement of the mineral matter of the 

 dentine may be regarded as persistent throughout its thickness. In 

 a transverse section of one of the tusks, an arrangement of the con- 

 stituent tissues of the tooth is displayed which leaves verj^ Kttle 

 room for doubt with regard to the aflSinities of its possessor (PI. X. 



