470 Geological Society : Mr. Nasmyth on the Structure 



organization of the individual tissues is the same at hoth periods, 

 though certain modifications of instruments are exacted at successive 

 stages of existence. Thus, in early youth, when the frame is not 

 powerful, every efficiency is given to the cutting edges of the dental 

 apparatus ; and the author states a fact he believes never before re- 

 marked, though long noticed by himself, that the tusks of the young 

 Elephant and Walrus are tipped with a very thin layer of enamel. 



The head of the Tetracaulodon Godmani examined by Mr. Nas- 

 myth is shown to have been that of an animal in which two of the 

 adolescent teeth are well developed. The crusta petrosa of the tusk 

 was about half a line thick, and extended over the whole of the visible 

 surface. The corpuscules were irregularly disposed, but closely ag- 

 gregated, and exhibited in the transverse section an irregularly circu- 

 lar shape with occasionally angular points. The radiating fibres were 

 numerous, ranging in all directions ; and the independent transverse 

 fibres were also numerous, traversing with a curved course the whole 

 substance. The cells of the interspaces were visible. The enamel 

 on the upper tusk was a line thick. The parallel rows of constituent 

 cells throughout the external half ranged in straight lines, but 

 throughout the internal half they were curved diagonally. There 

 was no clear space between the enamel and ivory, but the line of 

 junction was well defined. A plumose layer of fibres, apparently the 

 peripheral termination of the main undivided fibres of the ivory, suc- 

 ceeded to the enamel. The component bulbs of the fibre were round, 

 but not often visible, and were best seen in the longitudinal section. 

 The fibres were placed at about the distance of two interfibral spaces, 

 and curved in the transverse section as well as in the vertical, but in 

 the latter direction slightly. A minute corpuscular appearance was 

 scattered over the substance, and the cells of the interfibral material 

 were visible. 



The crusta petrosa, enamel and ivory of the under tusk were similar 

 to those of the upper, except that the constituents were so transpa- 

 rent as hardly to betray any characteristic. The parietes of the cells 

 of the enamel are more defined in the under tusk. 



Besides the important characteristic of the thick coating of enamel, 

 the tusk of the T. Godmani presents manifest differences from that of 

 the other species, in the elements of each of the constituents. The 

 radiating fibres of the corpuscules differ from those of Mastodon gi- 

 ganteum in being given off equally in all directions : in the M. gigan- 

 teum the numerous independent fibres of the T. Godmani are also 

 absent, and the zones or belts of minute corpuscules in the ivory of the 

 M. giganteum are wanting in that of the T. Godmani. 



Tetracaulodon Kochii. — The tusks of this Pachyderm have only two 

 constituents, crusta petrosa and ivory. The crusta petrosa varies in 

 thickness, equalling in some parts an inch. In the vertical section 

 the corpuscules are irregularly oval and irregularly disposed at the di- 

 stance of three or four corpuscular diameters, and they give oft occa- 

 sionally many fine radiating fibres. Numerous independent trans- 

 verse fibres pass in a curved direction also throughout the substance, 

 their beaded or minute corpuscular appearance being very visible, 



